While I was driving on a 90 degF day, I heard popping and crackling noises coming from one of the rear speakers. It continued if I turned muted the radio. It started to smell like burnt plastic and the speaker started making a squealing noise and then finally went quiet after driving for 20 or 30 minutes.

Later, I inspected the speakers and determined that the rear deck speaker on the left (not the sub) was not working and it had a burnt smell. I could still play the radio.

Amplifier must have gone bad, causing speaker damage. Parts alone from Mercedes would be $1700 for the speaker and refurbished amp, so I’m buying used parts on EBay for $250 and replacing myself.

Pulling out the damaged speaker and leaving it alone was an option, but I expect the amp to start damaging more speakers if I do nothing.

Interfaces are proprietary, so 3rd party amplifiers are not an option.