i think he is wondering why it does that...i believe some states have now illegalized it to play during motion...so you have to like jerry rig and f around with the breaks to get the movie to play or somen like that...is that what you mean dude
For most of those units you just need to ground the wire - attach it to any metal surface of the body. If that doesn't work, try attaching it to the ignition wire. Subfanatic is correct in that it is illegal to have dvds playing in view of the driver. Just don't be stupid and don't get caught.
Most DVD players are designed not to play movies when the car's in motion. Especially if it's been installed by a professional. He should consider himself lucky that he's able to play them while moving at all, so I don't see a problem here. Unless, of course this annoys him so much that he's willing to learn about automotive as well as dvd player wiring, then tear the dash appart to re-wire the dvd player to get past this problem. If I were him, I'd rather pump the brakes and flip a switch...
Yes, almost ALL indash players come with a safety feature, where you can not play any type of movie or anything that will distract you while driving. Most will only let you watch a movie, when the vehicle is in park with the E-brake engaged. It is also illegal now to have any monitors in the front of the drivers seat, only behind. Now you can go to some shops, who will make you sign a waiver or will just do it on the low for you (which is what i did) its actually all in the wiring, you find the right wire, and your good to go.