1987 300TD Diesel Wagon, 225k. Tach doesn't work, as well as dash light, console lights, and interior lights (except dome light near sunroof).

I should say that the instrument cluster warning lamps and turn signals work, and all console buttons, such as hazard and defrost, do light up when pressed. Just none of them illuminate in their off state, which I believe they're supposed to.

I replaced the potentiometer/rheostat in the instrument cluster but didn't solve it. OVP is good. Replaced all fuses under the hood and all bulbs in both the cluster and the lamp housings. Nothing. I'm out of ideas, any help appreciated. I realize Tach and Lights might be separate issues but you never know...

This one sounds like it will take a wiring diagram to chase down. Do you have access to a wiring diagram?

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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I don't unfortunately, though I'm sure you're right that I'll need one. I was so sure it was the rheostat or the OVP based on other threads, but no such luck.