The amp needs two +12v power sources to work. One goes straight to the battery(fused of course), and the other is a low ampreage turn on trigger that normally goes to the head unit. Since you are not going to have a turn on trigger (also know as remote output) from a headunit anymore you need a +12 turn on lead that will tell the amps when to turn on and off. You can get this off of your computers power supply or the ignition of the vehicle. Depending on how you turn on your computer would probably dictate which of these sources you use.
How is your computer going to be turned on and off?