On either your stock amp or tower speaker amp if you have one there will be a connection that looks like the rca inputs but will be labeled "output", "preouts" or something along those lines. What you will do is plug the rca's for your new sub amp into the preouts from either your cabin speaker amp or tower speaker amp instead of the head unit. What this does is just passes the audio signal from the head unit through the amp. I hooked the rca's for my sub amp to the preouts on my tower speaker amp that way when I have someone riding I can fade most of the sound to the tower speakers but still get full output from the subwoofer. Below is a diagram from a JL amp so you can see what I mean about the rca inputs and preouts.