Not to step on speedyturbo's toes (again), #4 also turns on the dash lights and license plate lights.

I'm not sure about the pre-86 cars, but many of these cars have a 2 position fog light switch. First detent is front fog light, second detent is front and rear fog lights. I don't think the rear fog circuit is live in the pre-86 cars. Put a bulb in the innermost socket of the left taillight cluster and find out... if there is a socket.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD