I have a '97 F150 SuperCab. The rear seat doesn't have head retraints. My kids are getting tall enough that their heads can hit the rear glass. I'm looking for a solution, without buying a new truck. (yet!!).

 

If you do not have a slider a cheap quick solution is a plastic rear window and gluing/bolting on head rest pads. I have seen the pads somewhere, not the window.

I think the safest thing for the kids if the truck was rolled and the window came out (read chop off the head unless bolted in) would be custom bucket or bench seats.