Amp. and Sub install on a 2006 Jeep Gand Cherokee

I actually did this install last week. I ran the power wire down the firewall next to the break booster and down to the frame. I then ran the wire through the frame until it came up under the rear driver side door. Drilled a small hole and ran the wire under the sill panel and interior molding until it came out in the back where I wanted it. With having the cable tucked up into the frame I avoided having to drill into the firewall.

Your best bet is to actually drill a hole through the firewall and istall a rubber grommet to protect the wire. If this isn't really an option, you might try locating the places the wiring harness and spedometer cable go through the firewall, or where the shifter goes through the transmission tunnel. The main thing is to protect the wire from heat and rubbing against metal. Hope this helps :)