Some car GPS headunits come with 7" screens, others with ~6" or so, and most have the ability to connect an optional backup camera. By design they pop up the rear cam picture when you shift into reverse. On my JVC you can also bring up the rear view pic anytime via the function menus. All perfectly legal.
The only thing that is not kosher is bypassing the parking brake interlock that prevents DVD video playing whilst in motion.