Like the other posts said, check the fuse first. If it's not blown, try replacing the rheostat. It's easy to remove. Pull off the headlight and the light dimmer knobs, pry/pop off the black backing plate and just let it dangle -- no need to disconnect/remove the rear fog switch. You'll see two screws (I believe) holding the rheostat/dimmer in place. Pull it out, replace the wires, tighten the screws and then put everything back and you should be good to go.

The rheostat that I've fried was pretty messed up. I looked at the metal coils of the rheostat (at brightest position) and it was black/burnt. Should cost you no more than about $5 at a yard and probably will take about 5-10 minutes to get it fully replaced.

You could just connect both black wires together to bypass the rheostat entirely, but I prefer to keep my car with it installed.
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Regards,
Eric Staufer,
'89 244DL 118k
My 240 Page