Take out the ashtray and try jiggling some of the wires in back to see if it causes the speakers to go in/out. It just sounds like a bad connection somewhere - but it could be much more complicated if your car has an active bass system with amps in the trunk or rear seat.

Edit: I'm not sure if 190Es had the same audio system as the 300E, I misread the title of thread

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth