Well, if it's anything like my car, the wire harness is on the drivers side
under the carpet right next to the sill. If I was wanting to run extra
wires to the back, I'd probably peel back the carpet and run the wire along
with the existing cable bundle.

If you buy an amp wiring kit, I imagine they have all the wire and
connectors you need. In my case I drilled a hole through the firewall,
RTV's a rubber grommet into place, then ran the wire through the firewall,
along the door sill, under the back seat and then to the back.

I have a 100 amp fuse on both ends just in case something happens to cut
the insulation on the wire.

Can anyone tell me how to route power from the battery to the trunk of
a 1990 Honda Accord?