For what it's worth, I bought the rear seat back from a Grand, ripped it open and removed the rear headrest brackets by cutting the welds with a grinder. I then removed and opened up the seat back from my Cherokee and bolted the brackets to the metal seat back pan. I had to make up some custom spacer blocks out of wood to allow the brackets to sit nice. I bolted it together using self locking nuts (nylon inserts) so they won't rattle free. I cut back the foam a little so the cushion would lay nice.
Will it hold up in an accident? I don't know but it has to be better than not having it. Even if it gives way, it will have taken some of the force of the acident.
In hindsight, the only thing I'd do differently would be to spend more time on the spacer blocks. My blocks are flat which means the headrests come out of the setback parallel to the seat. Ideally, they'd pitch forward a bit to be closer to your head.
EVERYONE should do this mod!!