Usually those headunits have video ins through RCA. It is not going to be a vga screen for sure, so just get a cheap vga to rca converter and then run an rca cable from laptop to HU. Only thing is if your unit is touchscreen, you cannot control the PC with the HU. You will need a mouse/keyboard so you can tuck away the laptop. And might as well get a dock for the laptop so it is easy-in/easy-out.
And how are you powering this? There are dc-dc units that will do a good job of powering your laptop, as well as an auto plug made for your laptop that you can just plug in/unplug. Or you can wire that to a relay so it is only on when the car is on so you don't drain the battery.
Whoopsie.... Now you have a carpc.
