I drilled a hole right behind the battery tray - you can get at it from the inside of the Jeep, below the glovebox. You'll see the firewall right in between the HVAC components there. Be sure to use a plastic grommet (usually included with an amp wiring kit) and seal your hole up with some silicone. Also be sure to split loom any cable under the hood. This location is nice because it's near the battery so you don't have a lot of cable exposed to the harsh underhood environment.
Inside of Jeep:
Under the Hood: