The switch is the likely problem. Pull it out and have a look at it. You can also remove the passenger side's working switch and see it it operates from the driver's side. Sometimes you can clean the pin switches out, sometimes not. I would start by working the switch with my finger fifty times or so while watching the light. Then I would try the pass. side switch.
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick