It shouldn't be too hard to find the hole in the firewall.
Lift up your hood, and trace the wires from the battery and you'll notice that they go through a hole, which usually has a rubber grommet on it, and that's where you can poke it through. It may be full, however, in which case I advise you drill a hill within close range of that hole, so your amp wires aren't causing too much interference with the motor.
Or if you can't find it, look under the dash on the driver's side (a flashlight would come in handy here), and trace any wires backwards (towards the engine).
Either way, it shouldn't be hard to find. And its not out of reach...