Start by testing the driver's door switch itself with a voltmeter, because that is easy.
Based upon posts, seems that the most likely culprit is the 'seatbelt buzzer relay', which actually performs more functions than advertised.
Bought one, but I have still not gotten around to installing it, so this is 2nd hand advice!
This relay is located behind the gauge cluster, on the right side (LHD), way in the back. Some prior posts even have pretty pictures, if you can dig them up.
Now the rear door deal is a whole other matter!
On my car, the rear doors activate only the rear light. Start with the bulb for this one.
1986 300E 5-Speed 240k mi.