Pull out the switch and have a look.. If the plastic tabs aren't broken you probably can lube it up with WD40 or similar, I did this on my W111 and it worked like a charm afterwards.

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Tomas, Sweden
1966 Mercedes Benz 230S with OM617.912, automatic. Disk brakes from W108
1983 Mercedes Benz 300TD grey, OM617.912 and 5-speed manual
1983 Mercedes Benz 300TD blue 7-seater, OM617.912 and 5-speed manual