Hi Everybody,

I wanted to tell you all about the sound upgrade I did in my W212 wagon from 2010. I bought this car on November 2024. Previously I had a 2007 W211 sedan. At purchase, both cars only had the standard sound. It’s okay, but it lacks bass and I always like to also have sound emanating from higher up behind me, so I’m surrounded by sound. The standard sound option only has speakers in the front and back doors, and no subwoofer. I upgraded the sound in the W211 in a similar way so I did the same in my W212 wagon and man does it sound nice right now. Deep punchy bass and nice crisp higher notes higher up in the rear so I’m surrounded by sound.

If you have some experience fiddling with cars, and you also want to upgrade the sound, maybe you can use my experience.

What I did was, I researched what speakers you have when you have the Harman Kardon sound option. Also speakers in all 4 doors, but also all the way in the D-pillars and a subwoofer. So, I ordered a subwoofer and speakers from e-bay, all second hand. These parts don’t have to look new, as they are covered by trim pieces.

But it turned out, that my D-pillar trim did not have any speaker grills, so I also ordered some second hand D-pillar trims that looked as good as possible for a reasonable price.


Just like in my W211, I powered all the speakers with a 4-channel JBL Concert A704. This is an amplifier with high level inputs. This is needed, because the amplifier needs a sound-signal, but you cannot get a line output (with RCA plugs) from the default sound system anywhere. You need to tap into the wire harness of the rear door speakers to get it.

Here are the part numbers you need:

D-pillar speaker: A2128201902 (2x)
Rubber mounting plugs for the D-pillar speakers: A0009934460 (4x)
Subwoofer: A2128202202
D-pillar trim piece left: A2126906125 (these are grey ones that I needed)
D-pillar trim piece right: A2126906225 (these are grey ones that I needed)
multiple meters of standard speaker wire
JBL Concert A704 or any 4-channel amplifier with high level input.
A car amplifier wiring kit. I had the Necom CK-J20 (5m cable is just long enough)
Plastic nuts: A0039900251 (2x) (to secure subwoofer)
Bolt with big washer M6x15 (to secure subwoofer)

What did it cost me:
D-pillar speakers: €20
Rubber mounting plugs for the D-pillar speakers: €6
Subwoofer: €114
D-pillar trim piece left: €27
D-pillar trim piece right: €27
multiple meters of standard speaker wire: ±€20
JBL Concert A704 or any 4-channel amplifier with high level input: €134
A car amplifier wiring kit. I had the Necom CK-J20: €25
Plastic nuts: A0039900251 (2x): €5
Bolt with big washer M6x15: Had it lying around

Total: €378

I started with the +-wire that runs from the amplifier to the trunk. Behind the battery in the engine compartment, there’s a plug that’s easily removed. You can drill a hole in it, in which the +-wire can be fed through. Pictures all the way to the bottom of the post.


You can pull the wire inside, and run it along the right side of the car. Pull away the door sill and the weather stripping for the front and back door. You can push the B-pillar trim out of the way and pry the cable behind the B-pillar trim so you can pull it through at the rear door. Then run the cable under the back seat. Behind the back seat there’s a piece of floor panel that you can pull out. Under that floor panel, you can cross the car to the left of the car.

Oh yeah, I placed the amplifier all the way in the rear left of the car, behind the panel with the netting on it. See pictures


Next is the signal wire for the sound. You need to take out the radio and tap in the wire harness behind the radio. There’s probably another possible place but I did it here. In the wire harness theres a quite normal looking ISO-radio-connector. The rear speaker wires are:

purple - brown/purple = left rear speaker
white - brown/white = right rear speaker

(in my pictures you see almost only orange wires. That’s because I have built in an android auto unit with a piece of extension wire harness with orange cables, luckily in this way I didn’t have to cut the original wire harness of the car)


splice the wires and connect speaker wires to them. You need to take off the under trim piece in the footwell of the drivers side (left hand steering). With some fiddling, you can feed the wires from inside the footwel, to behind the radio. Once connected, you can feed the speaker wires along the left side of the car. Same way as the amplifier power cable. Getting these softer cables behind the B-pillar trim is a bit more difficult. I used a long BBQ meat iron that I first ran through and I pulled the cables through with it. Feed the cables under the rear seat and all the way to the back where the amplifier will sit.


Placing the speakers in the D-pillars was easy. Make sure you wire them correctly (polarity). Removing the D-pillar trim is not that difficult. I’ve included work instructions for this.


The speakers are held in place with rubber cups. These need to be pushed in the holes first, and then you can push in the speaker.




You can run the wire along other wires downwards. The cable for the right speaker needs to go all the way to the amplifier on the left. So, I ran it down, to the front and under the same floor-board panel were I ran the amplifier cable.

The subwoofer fits nicely in the spare-wheel area. But if you don’t have the factory Harman Kardon, the holes and rods that hold the subwoofer in place first need to be uncovered. You need to take out the carpeting of the spare-wheel area. Once removed, you can see exactly where the subwoofer can be fastened. When you turn the carpeting upside down, you can vaguely see markings on where to cut some holes.


After cutting, place it back and secure the subwoofer. I ran the cable of the subwoofer trough once of the holes I just cut, then under the floor board panel and then to the amplifier.


I noticed that there’s only 1 speaker in the subwoofer, but it has 4 connections. That’s because it’s a double voice coil speaker. It has both the coil for left and right. Just connect them both, it works fine.
1 connection is red (+) and black (-) and the other one is green (+) and grey (-).

The amplifier fits nicely in the compartment in the left. I secured it with some angled pieces of metal with some holes drilled in it.
The signal cables from behind the radio need to be connected to the high level inputs. On this amplifier, channels 1-2 have their own connection, and channels 3-4 have their own connection. I connected them parrallel, works fine.
The nice thing is, this amplifier turns on by the voltage supplied to the high level inputs. When there's no input, it shuts down automatically after a minute or so.

Rubber mounting cup placed for D-pillar speaker

white outlining on where to cut in the carpeting to be able to secure subwoofer

note the polarity of the D-pillar speakers

D-pillar trim with speaker grille installed

Polarity of subwoofer side 1

Amplifier secured in left compartment. More than enough room

polarity of subwoofer side 2

rubber mounting cup for D-pillar speaker to be mounted in these holes. Can't miss

Speaker cable for high level signal ran behind B-pillar trim piece. I pulled it trough with a metal BBQ meat stick. It worked.

D-pillar speaker installed. Obsiously second hand but covered by trim piece anyway

secure points for subwoofer

location of grommet for amplifier +-cable

wires behind radio. The large bundle on the left hold the wires you have to splice into


It took me about 2 days of making small adjustments in the crossover and volume knobs on the amplifier and now it’s just perfect!

 

Wow, nice mod. Thanks for sharing 👍

 

Thank you so much for this. My rear speakers went to **** a few weeks ago and I couldn’t find anything on how to remove the back panels to get to it. This saves me major money once I order new speakers. Thank you so much again