For the DVD units, the manufacturer makes the green wire to attach the the e-brake signal. That is the trigger that allows the DVD to be played.
If you want to avoid that, do a relay by-pass. That is what I did. Do a quick google search on AVH-4200 relay bypass. There are a couple of good step by step videos. It is the same procedure for the 3200BT. I did it on mine and it is super easy. Much easier than tying it into the e-brake.