Nothing in the schematics for this feature, so it may have been added ... if so, I would suspect it to be a switch mounted on the floor , activated via handbrake lever..
Possible a US DOT Saftey Import thing or very late model ??
Never saw one on 113. And I do have all schematics for 113s , right up to 280SL/8.
However, the schematics for 108/111 do show a parking brake sw that ground the same circuit as fluid warning.
Possibly added to SL, but never shown on schematics
??
I do not doubt there is one on your car as you had it fixed once before, so it would be easiest to just lift the rug and see if there is a grounding switch there..if so, it should be a simple lever adjustment to complete the grounding circuit.
You know the lamp is good [ that is why I asked about the door open ]
Let us know ..
"inquiring minds"