VW made it pretty easy to punch a wire through the firewall... they had a 1/2" or so hole with a rubber plug in it... so I pulled the plug out, cut a hole in it, slipped it onto the power wire, and ran the wire through. So then it formed a watertight seal around the cable.

Under-hood wiring, with fuse

Through the firewall

Firewall wiring, from the other side

Run through the back seat, into a distribution block, and out to the two redbull-can-sized caps and a big inductor to clean up any nasty ignition noise on the power line.   From the distribution block, runs one 8-gauge cable behind the seat, under the carpet, and up around to the inverter where the computer is mounted; see the computer page for the detail on that.