on my 5th gen accord, i drilled a hole through the metal firewall under my glovebox.
on my friend's V6 5th gen, the battery is on the driver side so I looked for that plastic plug where the wiring harness goes to the engine. Take a kitchen knife and stab that rubber piece; you should be able to get a 4 or 8 gauge wire through there easily with the help of a screwdriver.