I am weekend wrenching on a '77 240D and wanted to fix the non functioning fog lights.

I popped off the 4-pin harness connecting to the left headlight assembly and found the following.
Pin 2 is a brown wire, assumed to be chassis ground.
- 12V potential between pin 2 and pin 3 when headlights are on
- 12V potential between pin 2 and pin 4 when high beams are on
- No potential between pin 2 and pin 1 when fog lights are on

Pin 1 is the fog lamp wire and is grey/green. This matches what I am reading in my Haynes manual, see photo attached.

I then looked under the steering wheel to the light switch to confirm that the grey/green wire is attached. See photo attached.

so my question is: Do these light switches have a tendency to break? I dont see any relays or fuses between the fog lamp, the light switch and the battery.

Thank you.

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I have owned over the last 20 years some 15 W123 cars and have never had a headlight switch fail.