This may sound over-simple, but are your instrument cluster lights controlled by turning the trip meter reset button? I drove around for months in my 300D without cluster lights, assuming the cause was hidden somewhere in the bowels of the wiring. Then I read where someone mentioned (probably on this forum) adjusting the lights via the switch I refer to above. Lo and behold, that's all it was!

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Matt
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1995 E300 Diesel (Die Blau Frau)