If you are looking at the firewall (like the picture shows) there is a point in the far right hand side a few inches down from the top of the firewall where the vehicles wiring haraness passes though the firewall. I simply cut a small slit in the rubber boot and passed the power wire though it. I then caulked the slit in the rubber boot I made shut (obviously after running the power wire) with silicone house caulk so water would not get though the hole I made.

Snowboard guy: Your amp under the drivers side seat is way too small. You will never be able to here it. Oh and nice work!