I said this in another post, but using a relay is one of the best ways.

Just hook up Ground to 86 & 87 on the relay, 85 to 12v switched (amp turn on wire), and 30 to the parking brake wire. The relay will reset every time the head unit is turned off.

 

This is rough but it might help.

 

Most you can just ground like I did mine.

I was going to do a switch like above, but didn't want to take the time to do it. I'll get to it l8r.

 

If you do a google search on "how to bypass the parking brake" on your unit, you'll find either videos or post about this subject. I use a relay on my 4300 DVD and it works great! Like someone stated earlier, if you can view/watch your gps while driving then I do not see why you can't do the same with the DVD function as long as you just browse the video. I mainly use it to watch/listen music videos so it's more listening then viewing. Just be safe and careful.

 

For my Clarion NX501 just grounding out the parking brake wire didn't work. I bought a single switch relay, connected the parking brake wire to one end and grounded out the other end. When I want to watch a movie while driving I just flip the switch on the relay and viola! I then drilled a hole in the plastic piece underneath my cigarette lighter and mounted the relay switch.

Disclaimer: I have never watched movies/tv/etc while driving. I only wanted it disabled so that my wife and/or kids can watch from the front seat. It is against the law and rightfully so. Do this at your own risk.