Originally Posted by Midlife
Just a guess but as a previous M-B owner I'll bet it is for rear fog lights. My old M-B 240D had a place for such lamps even though it didn't come standard on the car. Maybe someone else knows?
I had not tried to pull the lamp switch out to the second detent. I had forgotten that. I parted with my 240D after 18 years of reliable, albeit slow, use many years ago.
You are absolutely correct on the rear fog light...haven't seen it on a US car but that is what the additional click is for.