First off the grounding the parking brake wire is the easy way to do it. Works on old head units only. With gps and navigation you have speed sensors that register how fast your going. Your going to have to by pass those sensors going to the dvd player for it to work. If someone tries the toggle switch post up how it works.

 

It's not about it being in park. You can put it in drive and it will continue to play until you get to roughly 6mph. It is controlled by speed via the Nav antenna.