I have the same problem with the seatbelt light.
Seems to be related to which front door is opened first
Have to start to car to clear it.
This is said to be caused by the seatbelt warning relay, which I have in stock, but still have not gotten around to replacing.
This part also may cause dome light to become inoperable, or at least I am hoping there is only on culprit causing both problems...
1986 300E 5-Speed 240k mi.