I have the same problem, right down to the puzzling behavior with the flashing and the passenger door. This would appear to be very common, based upon discussion volume here.
Test the door switches first (because it is easy to do so), but generally this is said to be a problem with the multi-function 'seatbelt buzzer' relay.
Bought one, but still have not gotten around to installing it, so this is 'second hand' info.
There are a number of good posts with a great deal of detail on this topic. Some even have pictures for the install process.
Relay is located deep behind the instrument cluster, roughly above the ignition switch.
Best of luck.
1986 300E 5-Speed 240k mi.