So, you're a bit cooky and decide that building a PC in your car would be a perfectly normal idea?
Great! you've got all the basics you need! All you need to do now is get creative enough to take the plunge.
My idea originally started with the desire to bring my whole music collection with me when going on holidays, without carrying busloads of CD's or having it all unsorted on a USB-stick (and there is this feeling of immense awesomeness when you tell people you've got a PC built into your car.
My v1.0 was a converted EEEpc (laptop) from which I disconnected the screen, installed a touchscreen overlay and extended some switches and buttons on the laptop itself. Next I hacked it into the main console of my car and was good to go.
While not being perfect it performed the job quite well (from a it-didn't-explode"-point of view).
All in all I think it was a bit of beginners luck that held it all together.
Added problem of having a carpc that is "easily" removable is that thieves tend to smash your window to steal the darn thing... Which is exactly what happened. I was quite shocked that it happened and my first reaction was to drive around for 6 months without a proper audio device in my car. Then I decided to give it another go and make the whole thing more integrated into the car itself.
This whole instructable is written in retrospect after having sold that car and secretly planning to hack the whole thing into my new car. Some pictures will be blurry because they are pictures I already shot and cannot retake. I do not intend this instructable as a step-by-step instruction, they are more guidelines to get you going.
I will focus on the LCD part, because that's the part that requires some real hacking and slashing to make it look good, the rest of the PC is down to wiring.
If any step is unclear, do ask!