I dont quiet understand what you're saying. You want to plug your computer into your car or are saying you want to plug a regular power supply into your car? In either case I dont see why you would have to take apart the dash to do so unless you are planning on installing in the dash.
It's difficult to give you a generic hookup guide for a specific car type. Most PSU's require a ground, a constant 12volts + and sometimes a switched 12 volt source.
What kind of power supply is it? What kind of car do you have? Take a look at the "Show off your project" forum and search for your car type. PM the installer and ask some questions. They might be able to help out a bit.
Originally posted by ghettocruzer
I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
I bet you even have not decided what HW config you are going to use. Am I right?
This is what you really want to begin with. As soon as you have that decided, calculate power requirements of your system and based on your result and on money you want to spend you can easily decide how to power it. Generally, you have 2 options: DC-DC PSU (so your PC will be powered by 12V of your car) or use generic 12 to 220V inverter (so you can use PSU you can find in any desktop PC). Both ways have their cons and contras. DC-DC PSUs are more effective but pricey, inverters are cheap but generate line noice that is hard to fight with and they are big so basically unusable if you want small efficient system.
I suggest you to start reading posts in this thread: http://www.mp3car.com/vbulletin/forumdisplay.php?f=11
and come back to us with some more specific questions. You will definitelly get more helpful answers then.
One more suggestion: before you post next time, think about the question you want to ask and use that question in way everyone can understand as your message subject (title of your post). Subjects as "Hi there", "hello", "newbie looks for help", "opinion needed" are useless, very uninformative, and such posts are beeing ignored by others.