i would have to say that i've done it in the back on my car more times than in the bedroom or anywhere else, pointers:
1. Make sure the back seat of the vehicle has enough space to move around.
2. Make sure to bring a towel, blanket, and cleaning items especially if the interior of ur car is black, like mine....lol
3. Park somewhere that you know is not a busy area and where cops wouldn't patrol....i live up in the boonies so i have no problems with that....