ORIGINAL: onepoint8tee

Best thing to do is to splice some female rca's into the back of your head unit into the rr and rl speaker wires. Ya know there are two plugs for the female rca's, and two wires come out of each one. One of those wires is the signal and one is a ground (for each plug) so one wire goes to the speaker wire and the other to your common ground on the back of the head unit and same with the other plug. So the each ground is getting spliced into the same wire on the head unit and the other two signal wires (right/left) go into the RR and RL wires on the back of the head unit. I know that sounds confusing if you need more help/detail search or PM vitamin B6 or badluckaudi and they will be more help. They helped me alot with my setup.
After the female rca's are spliced in you plug in the male rca's and run them to the trunk or wherever your amp is, make sure to run the rca's on the passenger side to avoid interference with all the electrical stuff on the drivers side part of the firewall area.