No need to drill the firewall to run your audio power cable through.

The battery is under the back seat.

Lesson learned!

if you are doing a system, I would suggest upgrading the wires from the engine to the battery anyways...which may require drilling.

No drilling required either way. There is a nice sized rubber grommet with a wiring harness in it just the the left of where the aircondition lines go south on the firewall. ( When You are facing the engine) I have my nitrous injection/ purge actuators wires and pressure sensors wires to the front air bag/struts wires going through it as well.