ive never had time or privacy to look for an existin hole in the firewall...where did you guys run wires from the bat. to amp? i have a 07 supercab

 

Straight down behind the battery. Then underneath the cab to a grommet in the pass side sill plate.

Otherwise there is a grommet behind the battery.

 

Diretly behind the battery, and then run the wire 1/0 G or smaller under the sill plate...keeps it away from under the chassis (dry, isolated, worry free) to the rear of the cab. Check out the "Big 4" upgrade photo in my gallery at:
https://www.f150online.com/galleries...cfm?gnum=10819

Good luck!