If I look at the '82 schema for the SC, I can see that it also includes the switch at the driver's side seatbelt receptacle, so that should be there.
Once you insert the seatbelt (before or after contact), the seatbelt light should not light up. If it is not inserted and your contact is on the two transistor circuit with a capacitor charging over the variable resistor makes the delay that keeps the seatbelt warning light on for a certain amount of time. Adjustable by moving the potentiometer inside the relay.
In my MY85, when the seatbelt is not inserted, the relay that switches itself on/off is used to create a buzzing sound. Maybe that is different in the SC version ?