It's interesting to import from Japan. Congratulations on your new purchase! I've had the same 5.5l version since 2015 and have been quite happy with it.
I'm curious about the AUX input in the glovebox, as I never found it. There's a 6-CD changer and a socket, which I think is for an iPod. What kind of AUX input is yours, if I may ask?

 

That's the glove box, aux hole on the far side.
Though I have the M113 R500 so could be slightly different.

Finding an R550 coming out of Japan is next to impossible, I saw more R63s on auction.

 

It's a 3.5mm jack port. I definitely don't have it.

I see that you also have a CD changer, why don't you try playing a CD to test your loudspeakers (and please the boy)?

I wonder if there is a way to remove the CD changer unit and use the free socket for some Bluetooth capable device...

 

I had just thought of that last night but hell if I could find any of my old CD's I haven't listened to one in probably a decade.
I remembered though that my Audi Allroad came with CDs in the changer and I left them in there! Started off with some Strauss and tried an Alternative CD Mix they had as well.
Sound was good on some of the classical but certain songs and the alt-rock stuff was not great so I'm thinking it's Center speaker up front or the subwoofer itself.

I was also thinking about removing the CD changer I'm sure there's a way it has a bit of wiggle to it so one day I'll investigate pulling it out even just to have more storage space.

 

If the CD player sounds good but the radio and aux sound bad, your head unit definitely went bad (I just have speakers that have stopped working). The MOST connection goes on a ring to all components, including the CD player, so the CD player feeds its own audio signal straight to the amplifier.

You'll need one of these to remove any audio component without ripping out the MOST system from the car: https://a.co/d/ivNvOpX. As I recall there can be up to 4: the amplifier, head unit, CD player, and bluetooth phone unit.

 

What does that do exactly?
Further testing and sometimes it's got some distortion on certain songs but it really feels like it's mostly on the bass line the speakers themselves don't seem to be crackly.
I wiggled the connections to the sub under the rear floor and made sure it was closed tight and for a whole ride to daycare pickup it was fine but this morning was not great at times again.

If it does happen to be the head unit that's fine I have a Sony XAV-AX3000 sitting around to slide in there, just ordered the trim kit on eBay for only $120 and the MOST adapter on amazon for the same price.
Head unit swap out should be quick and i'll just have to see how the quality is. I have a backup cam, dashcam and subwoofer to put in but I'll do that later.

Are there any other adapters or things to add for like steering wheel controls? I remember having to get an adapter on my WRX back in the day for the steering wheel controls to work on the new head unit.
Same thing with the subwoofer. I might just replace the existing sub since it's in the trunk area already but has anyone added a subwoofer on top of the HK system.
I did this on my BMW that also had HK Logic7 stereo and there was a special adapter some guy made online that basically was a 3 way splice with the right harness connections to tie in the correct wires for the sub cnonection.

eBay and Amazon deliveries will be a week or two unfortunately. In the mean time I need to get a wheel speed sensor and airmatic valve conntrol box to fix my ABS and low front end

 

It closes the loop so that the signal can be sent through. If the loop isn't closed, you'll have an interrupted signal depending on where in the chain the device removed was.

Trunk subwoofer is a 6.5" 2 ohm speaker; I believe it's actually the same speaker as used in the doors, just in an enclosure so it can give more of an oomph. If you use an aftermarket 4 ohm speaker, you're going to have to find a separate way of powering it.

Steering wheel adapter will definitely be needed, but not sure what would work. The Android unit I ordered had the adapter and programming built in already so I didn't need to mess with that.

 

Thanks yeah I looked at the part and figured out what it was after I asked
Good for troubleshooting later potentially but for now I'll swap the head unit and see where I'm at. There's always the chance that the wiring got banged around the previous owner(s) put in a radar, they had a crazy big rear view mirror with reverse camera screen built in, plus there's a cutoff switch to watch DVDs while you're driving already. There's another little transmitter with a blue light at the top of the window dead center I don't know what that is either.

I do believe the subs I had were 2ohm and I'm not sure I'll keep them either way. That's a future project hopefully in the summer when the weather is nice to put in the subwoofer(s). I may end up selling the enclosure I made with the speakers as i'll need to find a good spot in the back for one or two speakers and make an enclosure to fit, it was a fun project on the old BMW I wouldn't mind doing again.
I may just end up removing the spare tire and making an enclosure to place there. We have roadside assistance and I was reading online that the airmatic gives you grief when you try and jack up one wheel since it's always trying to level the car and you basically have to take off the battery to change a tire?
Seems more trouble than I would want to go through roadside so the spare is largely going to be useless I think.

Any other good spots anyone has done/heard of to place a speaker without sacrificing storage space?

 

I have not heard of airmatic being an issue, that would be news to me. I have had to use the spare tire on my R500 (Airmatic with ADS, like yours) and had no issues- just make sure the car is turned off and no one is opening or closing doors. ~50 mph is a hard speed limit on the spare, faster than that and it gets a bit wobbly.

Giant empty cavity with the exception of the HK system amp underneath the driver seat. Just a bit of a pain to get to.

 

Ah well either way I haven't put on a spare tire in like 20 years. Get a flat? Tow truck. Done.
I could conceivably remove the HK subwoofer and just extend the wiring to a single like 12" sub in a circular enclosure where the wheel sits.
Decent access to solder the wires as needed so theoretically it should be easy enough without running any new wires from the head unit...

 

Still haven't had time for pictures yet but I did end up with an ABS/ESP error along with the Airmatic error.
Turns out one of the wheel speed sensors is shot, every once in a while after pulling off a curb the ABS triggers briefly
The front end is also lower than the back but not super low, that ended up being the Airmatic Control Valve Block thinking the front end is at Max pressure when it's not.

Wheel speed sensor + control block ordered and arriving shortly to fix that up.
I've also noticed twice at a stop light there's a bit of rough idle, after continuing to drive it's fine and it rarely happens.. not sure what that could be?

Additionally I've had a few times the car seems to get stuck not going higher than 4th gear. Not usually a problem as I can do 100km/h in 4th no problem but slightly concerned I may have a second vehicle in a row that could need a TCM replaced.
Maybe it just needs a transmission fluid flush? Probably hasn't been done but can't be certain unfortunately.

I also saw a random Facebook post in a W251 group that talks about water leaking coming in under the wipers somewhere that could cause issues with the water buildup. As my old Audi had a similar problem with the drain hole for the sunroof which caused a failed TCM thanks to a puddle in the passenger footwell If I can get ahead of a problem like that I definitely want to.

 

Also an annoying thing I have is the tailgate opens fine automatically with the button by the drivers seat and the remote control.
I can manually close the tailgate just fine as well however the button only briefly worked.
I assume it's just the button itself/wiring to it I hope?

I also notice that sometimes more than once the hatch autocloses/latches while driving. Does the car do this more than once usually after you take off? I assume when the doors autolock it would ensure the tailgate is latched also.
Never have a light on indicating the hatch isn't closed or any interior lights staying on so I assume it's functioning as normal.

 

Unfortunately 2007 is part of the trouble years for the 7G transmission. If it hasn't had the valve body repaired/replaced it will need it done soon. I do know that when the wrong transmission oil was put in, it would shift rough.

Drain holes for the cabin air intake can get clogged and lead to water leaking and pooling in the battery compartment beneath the front passenger seat. Front SAM is also located there I believe. Common issue on the W251, W164, and X164. It's almost absurd how many issues carry across the three models.

Tailgate should only latch and lock once. I would be concerned about water damage to the tailgate control module.

 

Well after having it in for a repair or two and also trying to track down another FOB I'm not sure it might be a 2006.
VIN shows early 2006 Build Date but Mercedes Canada at one point confirmed it's a 2007.

Drain holes issue was that under the wipers? In that area somewhere I think I remember reading?
Car inside is quiet, dry as a bone and no signs of water ingress anywhere so I can't imagine it's an issue. Yet
The latch I also assume is just something on the rear hatch door itself, it opens automatically just fine but the button to close does nothing 90% of the time.

Transmission has largely been fine as I drive it daily the 4th gear thing hasn't happened in weeks, most concerning is the ABS sensor which made it almost impossible to get up an underground parking lot ramp it was triggering the ABS so much. Turned off ESP and sat for a bit and it was fine, the part I ordered apparently was wrong so I need to get a new one.

Don't want that to happen again, was on the way out of swim class with the kids and the wife freaked, got her mom to come and tail us on the way home to be safe

 

Nice, at least it's working. My pseudo R550 has been down for a month due to a cracked radiator. Just got it replaced yesterday, was able to do it in the driveway. Almost everything is accessible from the top but there are two bolts at the bottom near the drain that needs to be accessed. Much easier done with the front left driver side jacked up, no need to lift any other sides. Turns out the OE radiator was a crappy skinny one- the replacement Nissens radiator is twice the thickness. Why my R500 had the base of everything mechanical while being highly optioned, I have no idea. On top of the skinny radiator, mine also has the 330mm front brakes and solid rear brakes as opposed to the 350mm front and vented rears the X164s and v8 W164s came with. Gonna replace them with the ML63/R63 brakes.... eventually.

 

Pull the trim off the liftgate to inspect for water infiltration.

 

Cracked radiators are no fun that's for sure.
The car is pretty dry all over so I'm not too worried though I'll look around when the weather here is better and I don't have so much on the go.

Lots to do we've got an easter egg hunt planned for a bunch of kids at our house. Hopefully after that i'll have time to put in the stereo and stuff.
I have the optical decoder box and the trim kit ready to go so shouldn't be too hard unless I have to take apart harnesses to splice wires.

Then I'll finally take some pictures to post

 

Still no pictures, one of these days i'll remember...
But thankfully Easter went swimmingly and since I had an extra day off Monday I got to install the stereo.

I was just doing the head unit install so no crazy wire running for camera or anything. I found a YouTube video that helped me identify some wires so getting the Battery line and the Ground was easy but getting an ignition switched power for the Accessory?
Sonofabish I could not for the life of me find any confirmation about the JDM head unit pin outs so after many hours I gave up and got a new 12v tester and found ... nothing. I could not see any switched power to use in the stock head unit which seems ... weird.

Thankfully since it was JDM there was a toll card reader under the dash they installed which I binned and used the wire set from that which pulled back to the head unit area without too much fuss. Voila an extra ground and switched power.
It seemed to take a bit of time for maybe the HK system to recognize the decoder box or something I dunno but there wasn't sound for a bit then there was.

I thought everything was set until after turning off the car then next time I came in, no sound. I thought it was my temporary splice connection using a wire nut so I got some butt splices and crimped everything back in to place.
Sound came back but again not for long. I dicked around last night wiggling connections and eventually got it again.
This morning no sound once again despite nothing being touched and the car not moving. As I was driving in our shipyard going the stupid 10km/h speed limit I was able to pop off the fiber connection, plug it back in to the decoder then sound came back right away.

So clearly it's fiber loop related or the decoder box is just bunk inside? I might be able to return as it was amazon, or get a replacement but that will take a long while as it came straight from China.
Maybe I need to get that loop tester to see if taking out the CD changer maybe helps? I don't think it's a wiring connection issue, the head unit turns on and off with ignition properly and everything works just no sound output...

 

I'm inclined to think it's an issue with the fiber optic box, especially since things seemed to be working fine before.

Yep, no switched power for the radio; it was turned on by CANBUS. The Android head units have a CANBUS decoder box for this purpose. I believe the decoder box also provided a reverse signal, steering wheel position for reverse guide, steering wheel button control, and also a signal for which door was opened so it displays on the screen.

 

Luckily I should be able to return it as the window on Amazon is open for 3 more days. Phew!
The decoder I got just had the fiber In/Out slot with a harness clip plus a connector that had a Ground, Battery, Accessory and RCA cables.

There is an existing backup camera and I have an aftermarket one already in case that one sucks but should the MOST decoder box also have more wires for other stuff?
The camera on the car actually fed to a large rear view mirror with a small screen section so presumably there's a wire somewhere I can bring down to the head unit area that can connect to the Sony unit.

I could care less about steering wheel controls for the stereo but definitely would want reverse position/steering line guides on the backup camera. Is that a separate harness or part of a MOST decoder in which case I should try another model of decoder this time...

 

The other functions are part of a separate decoder that interfaces with the head unit. Not sure if there is one that will work with the head unit you have. The MOST decoder wire connections sound correct- it only handles sound.

 

So finally some pictures! Sort of.
Unfortunately I have to wait for the seller to send me a new decoder box so not fully clipping everything back in to place as I have to fiddle every time I start the car to get the sound working.




Now since it was a JDM head unit it took some sorting to figure out. There was a 4 wire harness in the back with a couple of medium gauge wires. One was Ground and one was Battery. Not sure the last one but it wasn't Accessory.

One of the next steps will be replacing the subwoofer in the back. I'll have to build a new box etc so that will take a while but I'm sure some of these wires Ill need for subwoofer connections?




Also what in the dang heck is this seems like wires go from the front of the car and split to the subwoofer above and this unit.



I also need to clip the cover off of the overhead lamp unit area as there should be a wire that connects to the existing backup camera. I checked the old clip on mirror and there's a very small like 10pin round connector the other end should be up there...

 

How are the engine, transmission and axles on this car? What about suspension, steering and brakes? Any work done lately in those areas?

 

Keep an eye on that tranny definitely. I have an r500, it was our family car for a decade but once it hit the 100k mile mark the 1st one died, had it rebuilt and it died again. I was thinking of redoing it a 3rd time since i have twins that just hit driving age (actually brought them home from hospital when they were born in it), but after gutting the house 3x from the hurricanes here in Florida last season its more than i want to sink into it. I like what you did with the trim kit, got a carbon fiber niversal kit and a buddy at tint world and basically worked on it until it fit. The tailgate thing i know in the settings you can adjust the limiter and other stuff, one issue we had is that fuse box by the tailgate can get water in it if everything isnt sealed tight but it was an easy fix. Looks like you have about the same configuration as mine except mines the silver paint with gray interior. No clue how much itd cost to ship anything up there but if you strike out looking for any parts let me know. Im just going to part her out to other r500 ppl if anyone needs something. Everything is in pretty good shape and i even have most of the maintenance records since im particular with my vehicles. If i can help im happy to. And despite all its quirks, still think this was one of the best vehicles we owned and it did right by us for a decade. Hope you get as good or better results

Heres a pic of our r500

 

also ours was doing the esp abs thing as well. We had to replace the collar part on the steering column and a couple other things to fix it but as a temp fix if you can move that collar at all by hand (hopefully not but ours before replacement was like that) if you get the collar centered where its supposed to sit and stabilize it then turn it off for a minute or two and back on it would clear on ours. That was a pain. And i dont know if its same up there but when we needed a new FOB we had to go to the dealership. And i tried every key and programming buddy i had. Something to do with the security or something like that, i think it was just Mercedes way of making us go to them to drop a few hundred on a new key. Hopefully you have better luck than i did there