126 light switch light
 

I've been through the wiring diagram, but I can't figure out when the light in the headlight switch is supposed to come on. My bulb ohms out fine, but I've never seen it on. I've tried front fog, rear fog, standing, key on, etc. I assume its the same as a 123, too.

It is supposed to indicate "rear fog" (pull out 2 stops in "headlights on" position).

Up until some time in the 90s (near as I can tell), the FMVSS did not recognize/allow a rear fog light so this feature was disabled on US models. Then the rules changed and the feature started appearing on the US models.

My 83 240D had the 2nd position on the light switch, and an empty, unwired socket position in the left rear light rail. 123 and 1st gen 126 have the same light switch.

Same with my 87 300TD. 124 and 2nd gen 126 have the same light switch (which I think is similar to the 123).

My 98 E300 and 08 ML320 both have the rear fog light.

That makes the most sense as to why the light isn't shown in the wiring diagram, but I wonder why there is a light bulb installed if there's no way for it to work. Unless the ground goes through the rear fog light bulbs...

Same switch used all over the world, so it has the bulb whether it needs it or not.

The switch has the terminals NS (for the rear fog bulb) and 31 (for the internal light bulb ground). If you pull the switch and look at the connector plug, chances are you have 2 empty holes in the connector labeled NS and 31. You could run a new wire to the back for the rear fog light and a ground wire for 31 and it should work.