Dome light in '87 MB 560SEL

The dome light in my '87 560 SEL does not come on when the front doors are opened, although the lights at the bottoms of the doors do come on. The
dome light DOES come, however, when the BACK doors are opened. This sounded like a fuse problem to me, but all the fuses seem to be perfectly fine. Does anybody have any idea what might be causing this?
Thanks! Dan Solis

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