i dont have headrests in my car and i used a fisher price soft mirror, no hard parts, so that if it did come off in a crash, no one would be hurt.
what i did was sacrificed (at the time) my car upholstery and used sticky backed velcro on my car seat and the back of the mirror. it held up very well even through summer heat (worried about the sticky factor)
when i stopped using the mirror, the stickiness was minimal and used the goo be gone stuff to get the rest off.
**disclaimer** i have no idea if what i did was technically safe so if anyone with some more input wants to offer some i wont be offended.