Joe, The '99 to current Mustangs have the upper seat belt retractors in the rear panels. They are probably attached to some reinforcment equivelant to the roll/side impact reinforcement they put in cars. They come out of a slot in the panel and run through a loop/guide bolted on to the headrest that positions the belt directly over your shoulder. They have an S shaped slit to allow the belt to be dropped so someone can have access to the back seat easily. That would be hard to do in a car not designed to have the retractors inside the panels. Try a company called Julianos for aftermarket seat belts. They have a 3 point retractable system for around 30-35 dollars for each side. One end hard mounts at the top and runs through a sliding buckle and to retractable mount on the floor. The problem is that some seats don't allow enough room on the sides for the retractors. They are about 2.5 inches wide. But the system is already put together and you could mount the upper side on a rear panel with reinforcement underneath it somehow and run the shoulder belt through a homemade guide that attaches to your headrest. Good luck with this, I suggest that you just don't get in a wreck and you won't need them. I have a 3 point retractable system from Juilianos on my classic but it is a coupe and already had reinforcement ready. I won't need them because I'll soon have a 6 point rollcage to bolt my 5 points to but the Julianos were nice to have for reassurance.