We have the original speakers in our 1994 E320 wagon and have the same problem... one or more speakers go out regularly. When one goes out we turn up the volume really loud for a few minutes and amazingly the dead speaker comes back alive. Assuming it's some sort of electrical issue between the stereo and speaker, and not the speaker itself. The common fix is to get a new stereo and speakers and rewire the whole thing. It's not worth trouble shooting and repairing the original equipment, which is not that great to start with.

__________________
Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230