i guess i know my audio equipment... that happens when you live in a small rural town where a young man's social status is determined by how many eardrums his car can rupture...
the speaker wires are organized in pairs. they should be color coded. color and color with a black stripe. they should be labeled, either with a tag on the wire, or on the connector or the stereo. if they're not, the only sure way to know which wires go to which speaker is to test em. unplug them one pair at a time and play a cd. if you have to do that then be sure to label each pair clearly so you don't forget which ones are which.
you are going to have to splice the speaker wires. this is kind of a ghetto way of doing it, but it should work. if you have some extra time, browse around radio shack for a while and you might find some other hardware that could make a better connection. on to the ghetto rig!
if you got the stereo/RCA adapter cable i mentioned before, cut the RCA ends off and seperate the two halves (if you have 4 or 6 channel output on your computer, repeat this proces once for each pair of speakers you have in your car). each half will have two sets of wires inside. seprate those two wires and do the same to the other side. now to make sure it works, you'll have to get the polarity of the wires matched up, positive to positive and negative to negative. it might take some trial and error to get it right. strip a small section of speaker wire from the stereo and connect the wires, one speaker at a time to make sure you match them up properly. there isn't much power running through speaker wires when they're running so you won't shock yourself unless you cut the wrong wire. match em all up and wrap each connection with electrical tape and you should be good to go.
if you'd prefer a more difficult, but better method, stop by radio shack. you should be able to pick up some RCA connectors. cut your speaker wires from your stereo (leave about 10 inches attatched to the stereo if you don't have a built in connector) and solder the wires to the RCA connectors. if i remember correctly (you should probably double check this) the inner wire on the RCA connector is your hot wire. solder each connection of your speaker wires this way, going to the speakers, and then do the same to the stereo. get yourself those RCA y cables i mentioned and plug your stereo and computer into them, then to the speaker wires and crank up the volume.
there's a few other more profesional ways of doing it, but that should be enough to get ya goin. don't hesitate to ask the guys at radio shack, or check your town for a car audio store ask them for advice. heck, you could probably hire em to do it for ya, and then it comes with a warranty