Never the back seat, but if you recline the front seats
I've given up on doing it in the car, though, I keep getting hassled by cops.
One time when I was in HS, a police car drove by, and it turned around, than sneaked up on us from behind with the head lights turned out. My stupid @ss told my then GF to just lie still. When the cop knocked on the window and asked what we were doing, "I said we were just lying down"....
The cop said, that since he saw a bible sitting on the dash board, that he wouldn't write us up, but told us to stay out of trouble.