Some of the mercedes models have a separate fuse in the back of the dash cluster (like 190D's with the same modelyear). This fuse (if present) can be blown after an electrical short in the dash lightsystem occurred.
I have seen dash clusters with damaged "prints" (I do not know whether this is the proper word for the plate on which all dash components are soldered to)as well.
The potentiometer does connect to the small knob protruding from the gauges, corrosion however is unlikely.
The gear selection light is easy to replace...just to know for sure if this is the same or another (worn bulb) problem.
with a minor 600+k