There's a cutout diamond in the padding underneath the dash on the driver's side. It's even got a thumb tab to make it easy to remove. If you remove the cutout piece, you see bare metal underneath. That's the firewall. If you drill through it, you get right into the engine bay. Drilling is easy, the metal is pretty thin and there's not much in the way in the engine bay that you can accidently cut. You should drill from the inside of the car into the engine bay. I'm guessing 4-gauge amp power cable is about half an inch in diameter, so you can drill one big hole if you have the drill bit. I had to drill several small holes, then widen them with a bigger bit and punch out the middle metal. The edges are sort of jagged, so I have to get a rubber grommet.