The dashboard light rheostat is in the center cluster, underneath the letters "VDO". The switch you are referring to is either the speaker fader, or antenna control. The antenna control is typically spring loaded, as it acts like a switch. The fourth possibility is the fog light switch, which typically pulls out (but may rotate like a rheostat)....but this is typically left of the steering wheel near the headlight switch. I believe the sunroof switch isn't round in shape, nor is the rear seat light switch.

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All the best,

J. P. Mose
1968 250SL
1970 280SE 3.5 Cabrio
1987 560SL
1990 560SEL