It is used to manually move the seat if the motor malfunctions. I believe there should be a cranck that slots in there as standard equipment in the car but I don’t have definate details on this. Perhaps one of the other owners?
AHA!! Makes complete sense now. I figured eventually that it was one of the drive wires, but thought that was more of a “built in” thing into the seat.
In neither car that I’ve got have I seen a crank of any kind… good job the seats work 😉
I realise this is a bit late but could you use the complete electrical service manual for 1990-91 780, it’s the whole manual, prints and fault tracing.