This is the reostat that controls the dash lights... the wheel to the left of the steering column, next to the headlight switch. Since this effectively dims everything, it was most annoying when it turned itself off, like on the highway going 70 mph. The only two lights that were typically , but not always left on were the ambient temp gauge in the instrument cluster and the radio station frequency display, but occasionally those also went out. How this car is wired is completely befuddling to me. I've only tested it once so far and it seems to be holding. I'll let you know whether this was the culprit, or it's a loose ground wire as others have suggested. Hope this helps.
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John Shatzer, '97 V90 @ 110K