Best Place To Run Power Wire/firewall '08

[cbird1177711]

I am getting ready to install a new system in my '08 GT. Any advice on the best place to run the power wire through the firewall? ALSO, I have a shaker 1000 system. I have the metra harness needed and a voltage regulator, but when installing the new head unit, there are 3 connections in the rear. the main one and then 2 smaller ones. the metra harness has 2 connections, the main one and another one w/the rca's leading off it for the subs. what about the 3rd connection back there? also, when wiring the metra harness, i know that the voltage regulator is wired into the blue wires, where do the exit leads get wired into? Im sure i will figure it out once i get in to it, but just throwing it out there. now another question (sorry) i plan on only keeping the front door subs, and i want them to still be functional via the stock amps, and i am just taking out the rear subs. will this be a problem? thank you so much, anyone willing to help with ANY of the questions.!!