there is no "slingbox only" solution for anything, nevermind a car... slingplayer is just software, so you need a computer to run it.
if your video head unit has audio/video inputs, then you can use that as your monitor but you will still need a computer to run slingplayer on. the easiest/cheapest solution would be to use a laptop with a composite video output. check out eBay for cheap laptops (you can get laptops with broken screens pretty cheap).
good luck
