If there is weight on the seat, it senses that (probably a spring systme), and if the seat belt isn't in the buckle (there's a sensor there as well), the light flashes or it beeps.
Let's get one thing straight first off: you should wear a seat belt in the rear seat, as it significantly increases the likelihood of avoiding death or serious injury in a crash. That said, the law...
It is a legal requirement in most (if not all) countries that a person seated in the back-seat of a car or other vehicle wears a seatbelt at all times. This is to insure maximum safety for the...