You can trace the wires and see where they go - Compare with same cars in wrecking yard (almost impossible except to reconnect disconnected things.)

Or best is to find a wiring diagrams manual or pdf for your specific year / model car. The pdf for my 126 Mercedes is 164 pages long of just electrical diagrams for everything.

I don't know about for your car, but try searching google.com for...
mercedes 124 electrical diagrams

If you find a link with startekinfo.com, that is a Mercedes link and would be best - might take a bit of hunting...

If all else fails, try a Mercedes dealer ($$$$$$ Get out you wallet!) or alldatadiy.com ($$)