You just need to pull the knob straight out from the front to reveal the socket for the light bulb. It takes some effort to get it to pop off, and to do it without damaging the knob.

The bulb is a 2.1 watt bulb, Sylvania #2721.

I'm not 100% sure that the bulb actually is wired up on that switch, however. I've replaced it on both the original switch that was in my 1980 450SLC and then another replacement switch I have just put in and the bulb has never iluminated when the fog lights are on.

Steve

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'80 450SLC 122k Dunkelblau/Palomino