DVD head units have one extra wire in their wiring harnesses, compared to a standard CD head unit. On my Jensen, it's a peach-colored wire, and is the activation wire for the DVD feature. During the installation, the instructions call for this wire to be tapped into the emergency brake indicator light wire. The reason is this:

The e-brake indicator light is what is called a "ground complete" circuit. That means that the bulb always has power applied, but when you pull the brake, that puts a ground to the wire and completes the circuit, allowing it to illuminate. The head unit sees this ground, and activates the DVD player.

In my Jensen unit, I simply ran this wire to a ground (I used a screw near the fuse box), and I put a switch on the wire, so I could break the ground contact at will. Jensen calls for a procedure to activate DVD playback. In their instructions, they tell you to select DVD from the option menu, and when the "lawyer screen" comes up (the one warning you to park before you watch a movie), you release the e-brake for 3-5 seconds, and then reapply it. To simulate that with the switch I used, I leave the switch closed (activated), and select DVD. Then I flip the switch "off" for 3-5 seconds, and flip it back "on". This tricks the head unit into thinking I put the e-brake off for a few seconds and then put it back on, and the movie will start. Then I can drive away.

Basically, ground the wire that is used for DVD activation, and put a switch inline on it. Then you can break and reapply the ground in whatever manner is required by the manufacturer of your head unit. If there is no off/on sequence with yours, as there is with mine, simply ground the wire to a screw inside the dashboard and you're done.