One solution is get an FM transmitter which will take the signal from the DVD player output jack and broadcast it on an FM frequency so that you can play it through the existing car FM radio. I linked you to an example. there are a lot of these available. Try your local car audio place and see what they recommend.
I just installed a DVD player in our old chevy van for the long trips, kids love it. My question is , how do i get the player to run through the van stereo for the audio? Really do not want the kids to have to all wear headphones, thought there was a way you could do this without having to actually wire the player to the van stereo.
thanks!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic