It is part of the seatbelt interlock. If you are sitting, then the connection is closed, so the car "knows" that person should have their seat belt on. If it's open, then the car believes there is no one sitting there, even if it's the driver's seat. It was disconnected on my car ('74.5 260z) too. I don't know exactly what it does (buzzer, light, etc.) but that's what the FSM said it was.