Generally speaking, it's a pretty good bet there's a factory amplifier somewhere in the vehicle (probably behind the dash somewhere). If you can locate a wiring diagram for your vehicle, you should be able to hook everything up properly.
You might need line drivers, depending on the output voltage of your soundcard, but you can probably use the stock amp. You'll probably have to locate the remote "on" wire for the amp, and run it to your Computer PSU, and you should be gravy.