I have it working just as you want in my car, no problem. I have the AUX connector installed too which is how I feed the DVD in to the front screen and the TV on to the rear screens. In the UK, cars have a tv button in the back that the kids can press to change stations up, 1 at a time. for some reason they didnt think to make it go down! so up you go.

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X5 3.0D 2003
Mods:
Webasto telestart remote kit
Aluminium running boards
Tv in motion & Aux input lead, Ice>ink to Ipod 3g.
DVD rear headrest screens and dvd sytem integrated in to main tv system via aux lead (watch tv or dvd in front and back)
Homelink Gate Opener, Chrome Grille Kit, chrome tailgate handle, Chrome Guage Rings, Tunit V-CR 086
Standard:
Bluetooth, comms pack, sat nav, sports pack, sports seats, dimming mirror, electric folding mirrors, memory seats, heated seats.