Cannot find a path to send the power cable for the amplifier through
The firewall is the bulkhead between the Passenger compartment and the engine bay. Look for a rubber grommet in the firewall/bulkhead that has existing wires going through it and use this. Some firewalls have grommets that you can use to pass your power cable through from the battery. Remember to put an in line fuse on th live cable of the correct rating.