Most DVD systems tie into the e-brake or neutral safety switch of an automatic transmission. There is probably a wire running from the DVD player to the trans and/or e-brake. Disconnecting this or grounding it might do the trick. However, I'm sure there are aftermarket companies that make a better solution. I have not tried it, but I hear Allgig from Nav-TV might do the trick. Plus it has some added features, like Bluetooth and backup camera capability, that you might like. There is also a product called Lockpic, but I don't know anything about that.

Below is a link to the Nav-TV Allgig press release. The second link is to a PDF file, which is the manual for the Allgig. Notice on page one, it lists "video in motion" as one of its features.

Most video players for vehicles have a negative trigger to the ebrake to lock out video playback. If you ground this connection it MIGHT work, double check with a local stereo shop and be sure to word the question to make it sound like you are using this as a display in a store or something or they most likely will no be able to tell you the information. Some radios have a logic circuit in them where simply grounding the ebrake wire will not suffice, so be sure to put a switch on this lead and keep in your mind that this switch is now acting like your ebrake to the radio. Like my Pioneer unit in my truck, even when the circuit remains grounded, when the vehicle is turned off then back on, it realizes that the ebrake was not used then released, so it defaults to the message telling you that you can not view videos while driving. So to compensate for this, I have a switch hidden under the dash, that I turn off then back on, tricking the radio into thinking the ebrake is applied.