The sensor in each seat? (reacts to weight). The rear seats have a different warning and the light goes off.
*4: Rear passengers’ seat belt buzzer:
The rear passengers’ seat belt buzzer sounds to alert the driver that the rear seat belt
is not fastened. The buzzer sounds for 30 seconds after the vehicle reaches a speed
of 20 km/h (12 mph). Then, if the seat belt is still unfastened, the buzzer will sound in a
different tone for 90 more seconds.
*5: If the rear seat belt is not fastened, the light will go off after a short period of time.