in-car computer?

Anyone have any idea where to start on an in-car computer? I have almost no knowledge of cars, so if anyone could help me out I'd be grateful . I have some old components that I think might be able to stand the stress of an in-car enviroment. Old HDD's that i wouldn't mind throwing out, and a P2 system that has a high heat tolerance.

I'm guessing that the most expensive component would be some sort of LCD display, but I wonder if I could just steal that from a cheap GBA SP, but I have no idea how to blend it into the paneling though hahaha.

Also, I have no idea how I'd even power the components. Would a car battery last long on a very simple P2 system? I'm assuming I'd have to somehow be able to run a PSU through the car or perhaps a car already has the necessary 5v/12v rails?

I know I have a lot of research to do, but I have some free time and I wouldn't mind just wasting it even if nothing came out of it. It would be great to be able to make a kind of GPS/multi-media system/web browser in a car. The possibilities are endless, even if a laptop could do all of that

You can't use a display that's not prewired for video (Composite, S-Video, or SCART) or a VGA input. If you need an inexpensive display for displaying just media, I recommend an inexpensive Gamecube screen. Most of them include an automotive 12V adapter and accessories for exterior video inputs. I rewired one easily myself for use with a computer that has video output.

And for power you'll simply need an inverter to feed whatever smallish PSU you're using, or you could do one better and get a power supply specifically for your needs.
http://www.mini-box.com/s.nl/sc.8/category.13/it.A/id.356/.f