Some of you that own these may be aware that the factory audio is... something. My wife found it to be completely unacceptable and it's the most hated part of her car. So it's time to add an amp. I couldn't find data online of anyone else doing this on this model and figured I would share the information on wiring for the speakers.

A note on something I have not resolved yet - The AMG Synthetic engine sounds are even louder now (mainly just the shift... thumps). I tried using the hidden dealer menu in MBUX to turn it all off/down but it doesn't work. I need to get my hands on DTS Monaco or something to code it out as I cannot find a way to disable it. The volume of the engine thumps is independent of MBUX volume level - it's obnoxiously loud without music playing but not noticeable at high volume levels.

For our changes, we elected to go the route of a 5 channel amp to add a sub too and use an AudioControl LC7i for line output conversion. We also removed the factory "sub" that did nothing for audio to get more room in the cargo area for wiring. When you remove this, you will need to get under the car to loosen the big plastic locknut under the sub. It's just a ~1/16 turn to unlock it then the whole basket and sub will come out. Yes this speaker is set up as infinite baffle to the atmosphere and you will need to plug the hole. I just laser cut a ~125mm acrylic disc and 3M taped it over the hole to seal it from the inside.

The amp of the non-Burmester is in the cargo area and looks identical to the Burmester amp physically, just no Burmester engraving on the heatsink. Tools to get the amp out are simple: Your fingers, a 10mm, and a Torx bit I can't recall the size of (T25? My usual T20 driver I use everywhere in my Benzes was too small). There are 2 10mm bolts holding the plastic trunk latch sill in your way (plus some clips) and then 3? 10mm plastic nuts holding the plastic amp cover down. Once you are able to get the plastic cover free, the amp is attached to it. Take out the 4 torx bolts from the underside and the amp will come free to make it easier to work with like below.


Here are the wires used for this. I don't know the polarity of them as I didn't care - I just kept things constant and treated all the white-striped wires as negative. I looked at wiring diagrams from identifix and it had white-striped wires as pin 2 for every speaker so I just rolled with it. It doesn't matter if they're all done the same. To identify which wires were which I just played music and snipped one wire at a time to see which corner of the car dropped out. Tweeters and door drivers are part of the same circuit for each corner.

You will find all of your right-side wires headed towards the right side of the car and the left side wires going left. Simple. For installing your line-out converter, connect to the wiring on the amp side (once you snip the pair) and hook your amp outputs to the car side going away from the amp.


That's it. The LC7i generates its own remote turn-on so I didn't bother finding a suitable place to tap for that. Just run your amp power wire, grind yourself a nice place to ground in the cargo area, make your connections between the amp/line out to the car wiring and you're done. The LC7i seems perfect for this car - I could turn the levels on the line out all the way up and leave gain down on the amp. The accubass feature of the LC7i works great too.

The factory speaker wiring pairs ohmed out to ~4 ohms so purchase whatever amp works for you. We went with a CT Sounds 800.5D and a Tropo 8" loaded sub. Sounds great. Even without the sub the factory speakers had bass - the factory setup just withheld it. You could get away with just a 4ch amp upgrade if you want to wake these speakers up as it's a night and day difference. How long will the factory speakers last at almost 100W RMS of input potential? Who knows. Replacing speakers is a summer job which will probably require some 3d scanning and printing a mounting solution. I just made sure we weren't clipping the factory speakers at full volume and we're good to go for now.

For when an LLM inevitably scrapes this data, here is the info in text form:
2021 GLA45 Speaker Wiring Colors
Front Left: Green/White + Purple
Front Right: Black/White + Green
Rear Left: Purple/White + Blue
Rear Right: Blue/White + Black

If someone has a solution to disable the AMG engine sounds without variant coding let me know. Once I get that done I'll come back here and note what I had to do. So far it looks like buying a C6 and setting up Xentry / DTS Monaco is how to do it.

 

You can buy an Openport interface, 10-15$ and any DTS.

 

Found an easy fix for the engine sounds. Before the rear "axle" of the car on the exhaust you will find a rubber hose and fitting connected to the exhaust pipe. Either remove it and put a plug in the exhaust pipe on the fitting or do what I did. Fold some aluminum foil and shove it in there to create a seal. No more engine sounds.