On the stock/car wire harness are four wires. Yellow, yellow and purple, blue, and blue and brown. I can't remeber for sure but I think I remember yellow being the left side (don't shoot me if I'm wrong). The solid colors are positive, the striped are negative.

You can either cut those wires, or the wires that correspond to them on an after market harness. Just make sure that you get the positive and negative right. If you switch them you can blow stuff up.

For those that might nt know the center pin of a rca cable is pos and the shielding is neg.