Buy one that's not intended for car stereo use.

Really. It seems like they put a 200% markup on everything just because it's "mobile theater"...

For instance- One can get a 5" LCD designed for use with a PSone console for well under a hundred bucks. I've seen 'em for $50 used. They're available with 12v car adapters, and most of 'em take composite video or even VGA with some gentle modifications.

Here's a crap auto unit from Wal-Mart: Linkie.
$150 for pretty much the same thing.