Here's my plan:
Build a PC into my 2004 Pontiac Grand Prix GTP.
Keep the stock head-unit. It's a double-DIN 6-disc changer. No MP3 support.
Build the PC into the dash or behind the dash. Basically, not in the trunk. I want to know if it bursts into flames.
Without damaging the dash (much), mount an LCD touchscreen.
Build my own GUI for audio, video, GPS, OBD, and rear-view camera.
First, a few shots of the dash, to give some idea of what I'm working with.
In these pics, the cubby-hole has already been removed from below the HVAC controls.
And here is the LCD screen. It's a 16x9, VGA, 800x600, Shark SHK740. I was scared to buy anything from "gooddeals18.com", but it all worked out.

It's kind of weird at first, when you buy some piece of electronics, and it has now power plug -- just a red wire and a black wire.
Here's the dash, sitting on my couch.
I decided that the air-duct in the dash was my best bet for building a mount for the LCD.
The duct comes apart into 3 pieces. I removed the spinning globe piece (the one near the top of the image).
I had to cut a hole in the duct for cables to pass through. The cables for the LCD and for the GPS pass through this hole.
I took a hole-cutter and cut out a cylinder of wood to fit in the duct, and built a bracket onto that. Actually, I had to have a friend of mine cut out the hole. It turns out that a 14v cordless drill will not have the juice to operate a 3" hole-cutter for very long.
I used a hand-saw to cut the notch into the wood where the bracket would fit.
Eventhough the bracket is screwed into the wood, I applied some wood glue. It can only help.
I painted the face of the wood black. Well, actually I took apart a pen and used black ink. Then I drew a lip around the outside with a Sharpie.
Eventually, I decided to drill a few holes in the lower half of the wood, to allow air to flow through the vent. Also, you can see the GPS unit on top of my couch.
Here's the cubby from my dash. I forgot to take pictures of it before modifying it. Basically, I cut out most of the back of the cubby. I also cut a hole on one side for a fan to blow in, and mounted a 2.5 inch HDD on the other side of the cubby. I cut some ventilation holes in the top (over the heatsinks and fan), and I attached a piece of lexan to the top of the cubby.
This was done in conjunction with my work on the motherboard tray (also made from lexan). The idea is that the lexan tray will slide into the cubby, covering the motherboard.