If you look LEFT/ABOVE of the brake pedal, you'll see a white/square cover, behind the precut carpeting. You can first drill a pilot hole in the dead center, then about 1/2" hole thereafter. Make sure to cover the drill bit using electrical tape or similar to show no more than about 1/2" so it will stop it from going all the way through the firewall not to damage anything behind it. Then run a coat hanger through and up behind the battery cover and you can fish in the new power cable from the battery. Apply some veggie oil on the taped area of the fish so to make it easier for the reentry of the coat hanger plus the power wire. When you're done, apply silicon in the opening you just drilled so not to get air through from the engine bay into the passgener compartment.
EDIT:
To run the power cable to the trunk, first remove the LEFT most kick panel -- remove the plastic bolt by removing the center pin by prying it with a jewelers flat head screwdriver. Then the rest will just pop out. Next [IMPORTANT], remove the plastic runner on the driver's side door. You have to remove this piece first because there's a pin towards the dash, and if you remove the kick panel out first, it will break that pin.
You can remove the seat belt holder on the pillar by removing the bolt, then prying out the pillar cover so you can run the power wire behind it. But I just pryed the front side then fished the wire behind it towards the back. Next, the runner by the sliding door just pops right out when you pull on each end. Run the wire through, then towards the back. Jam the wire in using a narrow plastic spackling tool.
As per the ground, if you remove the Driver's side rear panel, right next to speaker, there's a factory post that's a ground. Remove the bolt and tab the new ground wire there. Hope this helps!
Is there any info on how to run power from the battery to the trunk for an amp or radio or whatever?