Well if you are going mobo to amp (even if it is through your original harness) you just need the left and right channel speaker wire.
Get a 1/8" phono cable and cut it in half. Now you have 2 cords each with 1 jack on side A to plug into your computer, and the other end 3/4 wires. If there are 3 wires than It is Left, right, ground. If it is 4 wires than it is left, ground, right, ground. Then just connect them to your wires that go to your amp.