adding amp to 2008 mustanggt w/shaker 500

Hi friends. I am getting a 08 GT and want to throw in my directed 600d and MTX 7500 thats been passed through a few cars and still sounds great. I have searched forums and had conflicting answers so I came here to the experts! I am just going to add the amp/sub to the existing shaker 500 system, can i get some help. A - where can I tap my LOC, some people say at the deck, some say you cant at the rear speakers leads cause there isnt a low freq signal there, etc. Can I get a solid answer with a wiring diagram/color code for how to achieve the best performance when adding in my LOC? Also, has anyone had any experience getting through the firewall on a stang, where can you recommend making my life alot easier, does anyone know of a good existing grommet to pass my 4 gauge through? Thanks for any help, of you have other pointers on grounding and or where to pick up my amp turn on signal as well that would be awesome! Elite

If its a auto theres a perforated part of insulation half way up and to the left of the brake with a grommet behind it. I just did a RS on one today with an auto so im not sure if there the same. It comes out behind the strut towerThe greatest revenge is living well and showing it

passenger side or drivers? Are you going from the inside out, I wasnt following exactly what you meant....any help clarifying would be great! Thanks for replying!

on the drivers side theres a grommet behind the insulation. If you look between the inner fender and the brake booster on the firewall in the engine compartment you can see it betterThe greatest revenge is living well and showing it