CLS55 AMG 06, SL350 06, 996 cab, all current. Gone : E500 W211, 2x M5s and far,far too many 911s
Just yesterday I had the same problem on my CLS. Removed the lower (speaker cover) panel . it just prized off. Three torque screws hold the speaker in and can then be pulled out. Speaker connection is on the top of the speaker. Mine wasn't even connected. Mercedes having failed to reconnect it after fitting a new keyless go door handle. All fine now. Took about 10 mins start to finish.
Obviously probably different access depending on model. Mine also has Harmon Kardon which may be different.
Mine was intermittent, and Carly for Mercedes was showing an error code for audio channel 1 open or short circuit. When I took the door card off it looked and felt connected but removing and re-plugging it has solved the issue, it's been ok for 6 months now.