Dan,
Welcome to the forum. The main fuse block in the engine compartment IS a good sourse for extra power with fuses. The diagram I'll attach to this reply is the fuse block with the spare fuses colored in pink. Not sure exactly which ones have power "all the time" but that's easy enough to find.
As far as going thru the firewall, just under the left side as your sitting in the drivers seat is a large groumet that has a couple of wires going thru it including the hatch release cable. That groument has enough space arouond it for several more wires. I used it for my Amp, undercar LED lights and my Ahooga Horn wires.
