Sorry I'm a bit confused as to why anyone would want to display the nav screen to the rear seat passengers, after all they are not driving are they! My kids would much rather watch a DVD of someone being decapitated than details of the road at spaghetti junction!

Anyway on a more helpful note I am in the middle of installing headrest screens in my second X5. I have gone for Necvox 6.5" widescreens and an Alpine DVD changer mounted where the OEM cd changer fits. You will need the Aux input to get the dvd audio in stereo though. So you will need a car that is 2002 or newer or you will need to update the tuner to the Becker version fitted from 2002 and on.

My major problem with the headrest screens was how to get the cable from the headrest screen into the seat without it showing. My solution was to machine a slot down one of the headrest shafts for the cable to pass through.
Looks very neat and OEM! If you are in the UK I may be able to help as I have access to a machine shop.

I will post some photo's of the headrest install as soon as I get a chance!