The rear window switch idea is good. That would work great if one uses the w123 rails.

I considered putting the seat switch inside the map holder, all the way in the front. I got the idea from the Audi A6 (C4). But I didn't want to lose any storage space, and wiring it would have been tricky.

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'82 300TD
'77 6.9
'75 280S
'74 280
'87 Porsche 944 turbo