Do the obvious and check the switches and power and ground first. Then you will have to go farther in your search. I have seen many dome light assemblies faulty, but you will have to diagnose this as the problem Do you have the diagrams for this chasis, if not, we have the service CD's in partsshop. Yours is a 124 chasis.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM