I'm not sure about the navigation stuff, but I can help with watching a DVD while your driving. Most car DVD players are grounded through the emergency brake. That means they won't play unless the emergency brake is on.
However, if you take a look at the schematics for your DVD player, you should be able to find the ground wire. Go ahead and ground it to another ground wire or the frame, and your DVD player will work while your driving.
Just keep in mind that this can be very distracting while driving, so try to keep your eyes on the road!