gl_runner

I'm on an expedition to retrofit the factory hitch in my x166.
I've gathered what I believe are all the components needed to pull this off. I've already scalped all the threads on this topic but was not able to find an answer.

What I am looking for and hoping someone here has the means to connect me to this information:

• Wiring Diagram for Front Wiring Harness (PN# 166 540 14 03)
• Any instructions/steps on removing interior seats and carpet to run above mentioned wiring harness.

Does anyone have access to STAR and can help with this information?

Thanks in advance.

marcd7tx

I did this on my ML350. Should be similar. There is a write up somewhere here on the maybe the W166 forums. GL Not too bad a job and does not require STAR, at least on my old ML.

Yeffsy

I just did this on my 2015 GL350. It had the factory hitch on it when I bought it, but not the wiring! The only wiring diagram I could find is this one:

I needed the "Trailer hitch wiring harness", the "Trailer recognition control unit", and the "Control unit wiring harness". It took me multiple orders with the dealer to get everything I needed and I had the interior out a few times. I was NOT able to find a wiring diagram that showed where the wires plugged into the "Rear fuse box", "Right potential distributor electoral connector (CAN)", or the "SAM control unit". So I installed all the wiring and had the dealer hook up the wires and do the programming. They charged me 3 hours of labor.

Jeff

gl_runner

Thank you - I did see that diagram. The only mystery is what pin to plug into the front SAM module. I know the CAN connection is available near the passenger side b-pillar.

Did you happen to have the part number of the wiring harness?

I had my dealer order the front "Control Unit Wiring Harness" (PN 1665401403) but the harness doesn't plug into the trailer recognition control unit. There isn't enough pin slots on the harness. Pic attached.

I was thinking of running the harness myself since the dealer quoted about $2,000 of labor. Do you have any advice for removing the interior panels, seats, carpet, etc?

Yeffsy

I got a similar price from my dealer. I had to take out all the panels on the passenger side, the middle row of seats, and the front passenger seat (I left the wiring attached to the front passenger seat because I was afraid of messing up the airbags). You're going to need the Trailer harness also. Once you have both harnesses and figure out where the trailer recognition unit mounts, you'll see how everything lays out. I spent a bunch of time trying to figure out where the bracket for the trailer recognition unit was supposed to be mounted, then found that I already had one. It's up on the side, below the back side window, behind the rear wheel well.

There are 4 or 5 wires that go to the middle fuse block that I couldn't figure out in addition to the front SAM. If you find a wiring diagram for that, please post it for the next person to try this!

Jeff

gl_runner

My harness already has the fuse block with the wires/fuse pre-installed. I was planning on referencing the fusebox schematic and correlating fuse amperage to figure out where in the fuse block would plug into. That said, I believe this is the wrong harness for my car because it does not plug into the the trailer module. The dealer has ordered a different harness (166 540 5035) that I'll try out. I'll be sure to post any tips/tricks/caveats that I run into once I get it sorted out.

That said, I have no clue where/how to start pulling the panels...

Yeffsy

The attached should get you started removing the side panel. You also have to remove the bottom bolt off of the seatbelt for the middle row.

Removing the seats is really straight forward.

I used a set of nylon pry bars to get the lower door trim off. Like these: https://www.harborfreight.com/nylon-...-pc-63594.html

Jeff

gl_runner

Thanks for this! - I was able to remove almost everything. I got the rear side panel, door trims, middle row of seats, and glove box.

I couldn't figure out how to remove the B pillar. I gave it some force but it wouldn't budge.

As far the front side goes, I was unsure how to remove the front passenger seat. I saw a Torx bit on the rail, but there also appear to be circle nail that appears to be crimped.

Additionally, can't figure out how to access the front SAM module. It appears I have to remove a grounding plate that is connected to the front fuse box and blower. I left this alone for now.

Hoping the second harness I ordered is the correct one for the vehicle.

Yeffsy

I think the B pillar between the front and rear doors. If you pull the door seals off you can see the metal clips. Those metal clips go over the edge of the sheet metal. Once you get the seal off you should be able to see how to get it apart.

There are 4 torx bolts that hold it down. Slide the seat all the way back to get the front ones and all the way forward to get the back ones.

That SAM module was not easy. You're on track getting the glove box out. There's a bolt or nut (I can't remember which) that you need a 10mm to get off. I put it back without the cover and it was easier. You may want to try to take the cover off first. You'll have to bend and twist and pull it, but it's doable.

Jeff

gl_runner

Thanks for all the info Jeff, it’s really helpful!

I was able to get the B pillar off and the seat unbolted. I found the CAN connector here. Thinking I can use this same highway to route the brake controller wire to the drivers side.

Question now is how to remove the seat belt and the air hose going into the seat? This seems to be crimped on pretty tight.

i can see there are some clips on the black piece that is covering the belt screws. What’s the trick to removing this? This should give me access to removing seat belt.

Car is going to get the hitch hardware installed tomorrow. Can’t believe the shop wants $550 to tighten a dozen screws. Oh well, it is what it is I suppose.

Almost there!!!

gl_runner

Got that plastic cover on the seat off, it just required some force .. now there’s a connector on the seat belt that I can’t figure out how to take off.

Mercedes has some great engineering on how everything is out together, but holy $&@$ ... have to remove everything to get to the wires...

Definitely do not want to damage anything and debating if I should let the shop take it from here and run the harness.

Yeffsy

I think your seats are different than mine. I didn't have to take off any ductwork to get the seat loose. I did take the ductwork off the top of the battery compartment, but that was relatively easy.

The hitch hardware was already installed on my GL when I got it, so I'm afraid I can't be much help with it. I remember seeing some threads about installing it and reinforcing it. So hopefully you have the information you need for that.

Jeff

gl_runner

Got the front seat removed and the SAM module cover off. Now just waiting for the harness, hopefully it’s the correct one.

Had the hitch installed by a shop the other day, and they forgot to hook up the wiring in the bumper for blind spot sensors. Does anyone know how to drop the rear bumper?

Meanwhile a sneak peak, trailer recognition module installed.

gl_runner

Had the vehicle coded today, dealer tested and confirmed all is working. Sounds like I plugged the harness in correctly! - Final check is to hook it up with an actual trailer.

L-L

Hello gl-runner,

Just out of curiosity, do you remember how much your dealer charged you for just the coding?
Also do you remember if you bought the OEM hitch kit 166.310.01.00, and if yes, what was included with it...just hitch and harness or anything else?

BTW, Both you and Yeffsy have very useful info here...

Thanks

Yeffsy

A factory hitch was a requirement when I purchased my GL350. It had the hitch, but not the wiring!!!

To install the wiring I purchased: 166-540-50-35 the wiring harness from the recognition module to the rest of the truck, 166-440-70-13 the harness from the recognition module to the 7-pin trailer plug, and 246-900-55-01 recognition module.

My dealer charged me $585 to connect the wiring (I removed the necessary interior pieces & ran the wires to their destinations without plugging them in) and update the coding for towing.

Jeff

L-L

Hello gl_runner,
I am trying to get the wiring harness installed on my 2015 GL350.
I am wondering if you had a BLUE plug that hooks up on the hitch trailer module? Are there 2 or 3 plugs going on the module (I only have the RED and BLACK)
Also in the F4 fuse box, how many wires did you have to hook up, 5 plus the blue fuse holder or 4 plus the blue fuse holder?
I received the harness from MB but it seemed to be a return so I am not sure if they send me the right harness. (No BLUE coupling to plug in to the hitch module and only 4 wires plus blue fuse holder, to install in the fuse box. (the wiring diagram I found online shows both the coupling and 5 wires plus blue fuse holder)
Thanking you in advance,
Lambros