Okay, Go to www.the12volt.com for your wiring needs. You need to register but then its free. (seems to be down at the moment) There are more out there just google. No one else said it so i will. You might consider a EQ Pre amp. It would have a volume control and would only need stereo input from the computer. Most of them would then give out front rear and sub.
I dont like the idea of the volume only being controlled through the computer. Even at home my speaker system has a seperate control for volume. And i think most EQ's have a remote out on them that could be connected to your amps. You might not want.......never mind you said no lcd. I was going to say have IE up or GPS running you might not want the amp on. But since you have no LCD
Option 2: External usb audio box with volume control on it. Mount it in the dash. =)