If there is weight on my passenger seat when I start the car, I hear a ding-ding-ding to remind me a seatbelt needs to be buckled. How hard would it be to do something similar for the back seats?
I have a Toyota that sets up a ding-ding-ding if you breathe too loud. I actually refuse to use the seat belt to avoid the constant dinging. I strap it across the seat and sit on it! Come and get me, coppers!