I'll agree it looks like an optical fiber.

On my 1986 W126 300SDL, one fiber goes in to the top, center hole in each of the following switches, left to right in the center console above the ACC unit:

Rear defrost
Air recirculation
(NOT the Hazard (red) switch)
Antenna switch
Rear dome light switch

Again, on MY car the fiber plugs into the switch before the multi-pin plug with the copper wires goes on. The plug then holds the fiber in place.

Your W201 may switches be different, but I expect that there are similarities in M-B's application of fiber. Look at your switches to see if you can see a hole in the top/center, above the plug pin holes. It may then be obvious which switch(es) get the fiber.

Best Regards,
Jim