Ok, here's some info:
If you dont want to destroy the integrity of your laptop, my suggestion is make use of the fun device called a "Power Inverter". Makes for smooth connections and such. If you are tuning your car, you will need a Tuning kit, it will quite often come with the software disc and cables to hook your laptop to your car, then you just place it in the passenger seat while tuning. For diagnostic and live info, your best bet is to pick up a particular device that acts as a diagnostic reader, that plugs into your laptop. Last time i saw one of those was when i worked for GM, and they had it from Snap-On.
Me, i'm working with an old laptop, the power adapter doesnt work, so i'm hard wiring it to my battery. I will also carve out the hole for the computer in my passenger side dash, and create a fiberglass box for it, cause it will be used for on-road web browsing by the passenger, and GPS/maps when needed. It will double as entertainment for drunk people too.
**NOTE: my car DOES NOT come with factory SRS airbags on the passenger side, only the driver's side. Vehicle set-up will be different for each vehicle**