...no voltage at the switch...
The bulbs in the dome lights are constantly powered and switching on/off is controlled by grounding the bulb...
I presume that the W126 S class dome light is similar to the W124 and later W201 dome lights...and these are known for their malfunctioning switches after several years of service.
You can get out the dome lights, dismantle the unit and clean the switch. If you are lucky it will bring you the requested light in the dark, if not...you might have to get a new dome light...
with a minor 600+k