Aftermarket Amplifier

Look for a place where other wiring is passing through the firewall and use that hole. You may need to expand it slightly and be sure to add an insulator around the hole's edge to prevent shorting with the wires (especially your new power lead). If no holes are available, you will need to identify a clear location and drill one (also adding insulation). Pull back the carpet and padding on the inside if it is obscuring your view. Locate a place you think is also clear on the engine side and, when confident, drill a small (1/8 inch) pilot hole. If this location is good, drill a larger hole that will accommodate your power wire.