Need some advice...

I have one child that is revolting against the seat belt. Seat belts are a #1 safety issue when they are in the car, but this one child has done everything from leave it off, to sitting on the waist belt and only using the shoulder belt. I have tried doing the firm brake press (not with anyone behind me of course), firm talks, and pulling over to the side of the road until they get their seat belt on correctly. Nothing seems to be working. So I need advice- how in the world do I get them to leave their seat belt on the right way so they are safe, 100% of the time?