Front seat belt relay switch
How to replace front seat seatbelt relay switch on a W123 Mercedes 300D
The W123 chassis covers 240D's, 300D's, 300TD's, 280E's, and several other models of Mercedes coupes, sedans and wagons from model years 1977 to 1985.
If you mean the relay that turns on/off the seatbelt warning light, and also the buzzer, it is located up under the kick panel beneath the steering wheel aka on the driver's side.
Use this guide to remove the kick panel on that side:
Then look for a silver box about the size of a pack of cigarettes, mounted almost all the way to the left near the driver's side door. This is the seatbelt relay. Note that there is another silver box, that is thinner but taller/wider, which is tucked up a bit further back in the dash. This is the cruise control box.
Once you've located the seatbelt relay, it's just a matter of unscrewing it and unplugging it and the reversing that to install the replacement part.