Most in-dash sat/nav units aren't allowed to play any video in most states. You can bypass/disable it, but that can run you into liability issues.
I had a SatNav where an "unable to detect park gear" warning always showed up (I had a stick shift, so it the wires were connected to the hand-break). The guy at the shop couldn't figure out why it was shorting so he just disabled it and said, "Don't watch DVDs while driving."
You know I never did use the DVD player in there ever, not even while parked, in the 5 years I had it.
Say you installed a media player. Now you'll have to convince a jury that you didn't install a media player just so you could watch videos while driving, and therefore were distracted at the time.