well mine was kinda trick lol,
the screens were free from a mechanic i know, he had taken them out of a damaged car, they had a specialist 8 pin lead, kinda like an s-video cord, so i opened the screens to see where the wires went, probed and then i spliced them, lukily it worked but splicng rca signal cables is never the best.
as for fitting them, they usually have a large mounting hole, so you have to firstly remove the outer case, mines leather then the centre section with the prongs for the seat, then the foam. trace the shape of the holder on the the foam and then use a blade to cut out, then refit the holder and the prongs to see if it pushes the holder out, mine did so i needed to chop it drastically, then you rebuild the head rest and cut the leather, corner to corner and fit it, but less is more deffo lol.
the last car i did, passat, simular headrest to a leon, i used alpine screens and they went in with no mods to the internal holder, but you can also buy full headrest screen combos off ebay, and then simply cover those in cloth.
my units do have a second source, but didnt even bother lol, but you could hook up a seperate source, but then you have to worry about sound sources or headphones lol