Mysterious switch on '82 500SEL

[filp]

Hi,

A friend of mine has a Lorinser 500SEL from 1982. Really nice car. It has the european AC system.
There's a mysterious switch that seemingly does nothing.
The decal on it is gone. The number on the side is gone except the lower number "(Lk) 03 8125 34". The female plug is 09967602 which shows up on eBay as a window defroster switch plug. But wiring is way different...there's a thick red and red/black wire, then another red wire from 3rd pin to 1st.
The pin layout on the switch itself is identical to a sunroof switch. There's also a thin black wire on the top of the switch not connected to anything.
When you press it, you can hear a crack through the speakers.

Does anyone have any clue what it may be?

Pictures: https://imgur.com/a/iQlQQuB

Thanks