In the pictures above you see my Head Unit playing a DVD. I can turn the main HU to the radio and have the rear continue to play the DVD I have in the HU. While doing this, the radio is played out of the speakers and those that want to listen to the DVD that plays. Well the TV's came with wireless head sets for both. So turn them on and it connects. Say the driver wants to listen to the radio, he can. Also the right rear passenger wants to watch the DVD in the HU, he can with the headset. And say the left rear passenger wants to play PS3 or something. There is a switch on the screen that allows the user to switch between, the main DVD player and 2 other things that could be hooked up to it. IE: DVD player, TV, video game system. Again all can be listened with the other headset. Both TV's have the switch to change between Aux1/Aux2/DVD. The main HU can switch between radio/DVD/Aux1.
Also say the kids in the back aren't listening to you while you drive. Key function in the front, you can go into the options part and turn them off.
I don't believe these screens can be hooked up to the factory HU. I could be wrong, but I don't know. For a sure thing, be best to check out the local stereo shop for ideas. But as Clay mentioned above that is another route. Some screens you might see on EBAY come with a DVD player built in, plus with speakers in them.
For whatever route you take, had you thought the idea of a ceiling mount single screen?