I've run into all three reasons (1) fractured solder joints on the OEM radio circuit board, (2) dirty volume control, (3) broken wires in the door hinge. Had all three problems in one car!
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Art Benstein near Baltimore

It could be the connections at the speakers. Pop off the speaker grill, remove the mounting screws and pull the speaker out. Check to see if both speaker wires are connected. (I had a dead speaker which had one wire disconnected.) If they are connected, pull each wire spade connector on and off a few times to wipe the connections clean. Also be sure the connectors are tight. If they aren't, sqeeze them a little with a pliers. If this doesn't do the trick, try substituting another speaker while you have the wires out. If all this doesn't work, you may have broken wires where they go into the door or the connections at the head unit may be loose.

I had the same experience on my '90 240. I check continuity in the speaker wires, since I've seen them break at the door hinges. When this checked out, I loosened the radio mounts, shook the radio a bit and discovered the source of my problem. Replaced it with a nice Sony from my parts car.

I had the same problem with my speakers. I sprayed some contact cleaner in the volume knob, which also controlled balance on my head unit, and that worked for alittle while. I found it much less agravating to just replace the head unit and have not had the problem return. Replacing the head unit took just a little while, so it was worth it. Good luck.

Pete '89 240 DL 236,000 miles.