If it is a car, attaching the wire of DVD 'double-din' unit to the park brake wire to ground wire would enable DVD playback feature but ONLY when the car's hand brake is on. Meaning you can only watch DVD with the car stationary and hand brake applied.
However, there is a way around this. With most of the older DVD 'double-din' units, you can simply override DVD in motion by connecting this wire to a 12v relay. Very simply process and it takes literally 5 minutes.
If you have a newer Unit, then you would need some type of an electronic chip override. Works exactly the same way as a relay but just a little bit sophisticated. You only need to connect 3 wires (DVD unit parking brake wire, ground and remote/amp turn on).
Depending on exact model of the radio, you can conduct a quick eBay search and it will show you exactly what you need. For example, look at eBay link below. This electronic override chip works on PIONEER, KENWOOD, JVC, SONY, CLARION and ECLIPSE stereo DVD units. You cant beat the price of only $9.99 including free shipping. ::
Originally Posted by Rolling Thunder
Usually all you need to do is attach the park brake wire to the ground wire.