There are two little twisty knobs on either side of your speedometer. One of them will reset the trip odometer when you press it. The other one will adjust the clock when you press + twist it, if I remember correctly. I think if you twist the knob that you use to reset the trip odometer (don't press it, just twist it), I think it adjusts the brightness of the instrument panel. Unless I confused it with the clock...anyways, just try...it is either one or the other...

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1995 black pearl/black Mercedes-Benz E420 :