I know this isn't a typical part of the CarPC installation, but surely some of you are like me and never get enough of this stuff. And of course these can always be used to explain the purpose of that DVD player in the dash.
I used TView 6.5" TFT Headrest Monitors from Ebay. I wanted some that were all black, and I already have a TView DVD that works well for me so I tried TView again and am very happy with the screens too. The seller was CMTTrading/JudysDeals and I believe I paid about $125 each. Don't waste your time with non-TFT screens - these look awesome!!
These are 2002 Altima headrests, but a similar process could be used for any headrest screen that is too big for the headrest foam you're installing it into!
Remove your headrests and the covers. Construct a balsawood frame for your headrest shrounds. I used 1/4" x 3/4" strips of balsa cut and glued 2 layers thick to allow for a radius beyond the edges of the shroud. Use medium CA, and stagger the corners for strength. I angled the bottom edge to make space for the seam in the upholstery and to better align with the bottom of the headrest:
Trace your frame on the back of the headrest, and cut away foam down to the tubing, and until the headrest fits just even at the bottom. Remove some additional foam in the center for airspace and wire space. Add a piece of 3/16" hobby plywood just wider than the tubes wedged and glued underneath to later screw your shroud into. Drill a hole into each tube to fish your wires through:
Sand some nice radiuses on your frame, and glue it in square. I used hot melt to glue everything in the headrest:
Align the bottom of your frame with the bottom of the headrest. Note the amount of foam that needs to be added in the top corners. I stuffed these after upholstering. I also added a shim of balsa to the bottom of each frame between the frame and the tubes for added stiffness:
The next step is tough. Pull the upholstery back over your headrest. This will take patience and time, but once you get it started it goes fairly quickly the rest of the way. I recommend another set of hands. Make sure that your frame is well constructed and securely glued to the headrest so you don't break a corner joint like I did the first time.

Then cut out the hole undersize, glue down the edges, then final trim the upholstery flush. Stuff the open areas in the top corners with ripped up little pieces of foam pushed in on the end of a small phillips.
Now reinstall your shroud, cut, fish, and splice your wires, drill your mounting pilot holes, and install it with the two center screwholes and some panhead wood screws. Plug in the screens, and they snap in place.