Get a scanner . . .
and read the DTCs in the car. That should give you a clue. If the wiring harness is very brittle, then that can cause all kinds of problems.
Re the door speakers: Remove the door panel and the speakers are exposed. They sit in a baffle so the size may be smaller than you think.
The panel comes off fairly easily: there's a screw in behind the handle that MUST come off.
Regards . . . . JimF
'94 S500 Cpe
Visit my Mercedes Web Page