Cees,
Virtually impossible to replace a bulb in the center cluster without taking it out.
Start by checking the door switch. It simply pulls out of the door jamb. Should be a 2 core wire on it with 2 pin plug. If thats in place and one of the wire is live (can test with a test lamp etc.), then it could be the bulb. Jumping the two wires in the plug should light up the bulb as well (showing faulty door switch).
From the brake fluid float switch, you can remove either connector and just jump the two pins in the plug - that should turn the light on (with ign on!) as thats exactly what the float switch does - connect those two wires.
Have you got light yet??
The float switch would have to be removed or fluid drained to ensure that it is operative.
Did your car ever have hazard flashers? I believe in those days, the hazard flasher unit was separate from the indicator flasher (and I think the flasher unit/hazard switch were one unit).
My '68 280 never had hazard flashers. I have fitted a Hella unit which is combined flasher unit/switch but the knob on it is not Mercedes looking, so I mounted it under the dash.
This any help?
naj
65 230SL
68 280SL