Well the easiest place to get through the firewall is under the passenger side dash. There is a large grommet with a wire harness there, just go though that. I have mine going through right between my brake master cylinder adn my clutch master, but its VERY tight clearence there. You must be very careful if drilling there if you have a manual trans(clutch master cylinder is there). For your amp i would suggest 4 guage wire, no smaller to be safe. Fuse that wire no more than 12-14 inches from the battery, with a proper size fuse. Remember, this fuse protects the wire, not the amp, in case the wire shorts to the body of the car..

13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
My webpage: http://eclipsed4evr.home.comcast.net
-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)