You need an fm modulater. Go to this page
http://home.comcast.net/~jarvis.wynn/ and about 7 pics down is a pic of a modulater. It takes rca inputs and puts them out through an antenna output that goes into the antenna input on your stock head unit. You'll need a 'head phone'(I think it's really called a 1/8 jack) to rca adapter(real cheap) going from the computer to the fm modulater. You ground the modulater and give it power, hook the car antenna's wire into the modulater and feed the modulater antenna wire into the head unit and you're done. To hear the sound you just have to go to the station that is designated in the modulater's manual. Need more help post up or pm me.