One possibility is a bulb that has shorted one filament to another.

Perhaps the easiest and cheapest way to check this is to get a nice new bulb or two and swap the bulbs one by one and see if the problem departs.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition