Correct. That other knob that looks like your trip odometer reset knob is the rheostat(dimmer switch). If your's is not on it's lowest setting, then it has failed. Some have removed theirs and soldered a wire between the two contacts. So as not to have a dimmer choice at all. Best to just replace it.
This involves pulling the instrument cluster out, removing a phillips screw that secures the rheostat, and installing another.

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Selling used Mercedes parts on Ebay...always changing...
cdplayer51 take a look!
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