You will probably find that if your sound card is sufficiently specced then you will have both front and rear outputs for surround sound. Use a 1/8" to twin RCA adapter on each of these (if there is space) and run 2 separate RCA cables down the car to the amp.
you can use y-splitters, but they aren't particularly conducive to good sound quality as the voltage of the signal is split between the 2 cables. Ideally you should be running some sort of line driver between the computer (roughly 2V) and your amp (needing appx 4V) for maximum flexibility and efficiency from the amp, and therefore greater quality.
Hope that helps