where is the best place to run an 8gauge amp wiring kit through the firewall on a 2008 malibu ltz. all the existing wires run down a channel alog the passenger side, I am hooking up a rockford fosgate punch 300 1"2 with 300 watt amp, from what ive read I can splce a 2 channel line ouput into the yellow,brown,light blue,dark blue wires running into the factory amp (splicing pre amp) for the rear speakers any other advice would help out greatly Thanks!!!