If you search for amp or amplifier install, there isn't any info about penetrating the fire wall. However, there is a great tutorial from Clipsed on how to install foglights and he covers how to get through the fire wall.
http://www.nicoclub.com/articles.php?id=262175However, I tried tried to do it this way and I couldn't get a 4 gauge through the big rubber grommet. I actually removed the battery, removed the stock air box, unbolted the ECU and moved it to the side (without actually unplugging it) and was able to expose a large enough area of the firewall to drill a hole. There was actually a small spot that was already depressed (like it could be punched out for wiring if necessary) and I just drilled it out. Sorry I don't have pics, but hope that helps.