Many ways to do it; here are a few:
Run output of your soundcard to an auxiliary input on your head unit if it has one. If your head unit does not have an auxiliary input you can use an RF Modulator that plays through the FM station on your radio. Also, if you have a tape deck you could run the output of your sound card to one of those cd to tape converters. Those are relatively cheap methods of doing it. More expensive methods are running the audi output through a USB sound card straight to your amp (if you have one).