Hi,
in my 1985 model, the seatbelt relay is 91161810200 and it is located behind the second gauge from the left, in a double socket (there is another relay right next to it, I believe the relay for the electric window motors 'Fensterheber' ? - see photo).
This relay actually does not switch anything on/off except itself and creates the 'buzzing' noise that way.
The Red/BLACK (RT/SW) cable is the +12V and also connects through to the warning light block, the 'L' output controls the seatbelt warning lamp (GRN/YEL). The electronics look like a simple delay circuit.
The other lights in the warning light block are controlled by the handbreak switch and the continuous circuit through the 4 brakepad wear sensors.
This relay has NOTHING to do with the rev limiter, that is a set value in your Motronic EPROM chip. If you have an aftermarket chip, they often set the revlimiter to a higher value (you can use TunerPro and an EPROM writer to read the chip / change value / write new chip).
