DOH!!! That really stinks.... Usually, there are holes in the firewall that are already there, but filled in with rubber gromets. On my Grand Prix, I ran the power cable down the passanger side, and out of a hole on the SIDE of the fire wall that was shared by the radio antenna. Worked like a charm, and thankfully, no interference with the radio's reception.
I can't wait to get back into the car audio game... I still have my old amps, and the wiring, but the quiality is less than that of what I would put in my car now. I had 2 Jensen amps, a 500W 2 channel, and a 600W 4 channel. I also had a Jensen bandpass sub box with 2 10" subs in it. It was pretty nice at first, but over time, the shortcomings started to get to me. The sound quality wasn't all that great. One of my friends had a $3000 JBL system put into his Couger, and that thing was SWEEET... totally blew mine away. Another friend had an MTX system in his Celica, and that was really nice too.
Anyways, good luck with everything on that!