There is a rubber grommet in the firewall on the driver's side behind the battery. If you stand in the front trunk and look toward the trunk, it is a few inches to the right. Removing the battery will make everything a lot easier. Make a small incision and push a long screw driver through, it will exit pretty high up inside the driver's footwell. The screw driver will help you find it from the inside, because the grommet is behind a layer of insulation, so you will need to tear a small opening. I would highly recommend putting some sealant around the wires when you are done to avoid leaks.
There is another rubber grommet between the battery tray area and the interior of the front trunk, same process, but to see this grommet you will need to remove the spare tire and significant amount of the front trunk carpet. I don't remember the exactly location of the frunk grommet (there are a few of them), but I think it is behind where a CD changer would be installed.
Best of luck!
