If you let me have your car details, e.g what spec is it. from this i can tell if it has got CD, satnav, what size screen is fitted. i have some wiring diagrams for the P12.
As for using the screen for multimedia device's, it may be possible to link into the rear park system (camera if fitted) as this produces the relevent signals to the screen. will have to look into what recieves the infomation and how, and if they are connected through DIM leads or not.