You can't wire directly into the radio harness from the pc, because you will need something to power the speakers.
Make the FAQ section at
www.mp3car.com your best friend

You can find tons of information on this subject.
I'm using:
Jetway Mini-ITX motherboard
AMD 5050e 45w CPU
2GB Ram
Onboard sound/video
64GB SSD Drive
BU-353 USB GPS
DSATX Smart Auto Power supply
Windows 7 32bit Ultimate
Centrafuse 3.5 Front End
Xenarc 705 7" touch screen
I'd assume like me you will ditch headunits all together. This means you need an amp to power your speakers. I installed an Alpine PDX-5 5channel amp under my drivers seat, and the computer goes under the passenger seat. Wire all your speakers with speaker wire to the new amp, and from your laptop you can take 1/8" audio to RCA splitters to the amp inputs to get sound from the laptop to your speakers.
The only challenge for the laptop end of things is powering it. My DSATX allows the PC to turn on with the key, and is set for a 10 minute delay to power it down (stop for gas it stays on, stay at home it shuts off). It can even be set so that your computer enters hibernation and powers back on with the key, and it monitors the voltage of your battery and doesnt power it down completely until your battery reaches your preset voltage (leaving enough power to start your car).
One of your most important choices will be your front end. Can't just use windows to operate your pc, it just isnt 7" touch friendly. I used Centrafuse because it was user friendly, but there is a free one out there called RideRunner (RR).
Check out these products for powering your laptop.
http://store.mp3car.com/Automotive_D...ators_s/11.htm
I was excited to see this thread when I saw the title. I've been off and on working on my project for about 6 months, and just got my pc temp wired back into the car to test. It is definitely a time consuming and sometimes frustrating, but overall fun project. So far mine is back in and powering up/down and running music just fine. Now I have to ground it better and do the wiring more permanently so it doesn't whine for 30s while the PC boots up till the 3.5mm jack is active, find out why the GPS decided it doesn't like to connect to satellites, and get it to play nice with my iphone 4