Actually, there is a perfect passage through the firewall on my '13 (almost certainly on previous years, too). On the passenger side, take out the glove compartment. There is the rear washer fluid hose going through the firewall, and just 1 inch up diagonally is an unused 1/2inch hole covered with a rubber grommet. Pop out the grommet, slit a hole big enough for your wire (mine was 6ga for the new Power inverter) and route it right through there. It's just about hidden behind the front wheel well under the hood, but easy to feel and push the wire right through. The grommet will act as the perfect protective insulator so as not to rub through the power feed wire over time.
I used it to install the new 1500 watt power inverter and continued the same source to the trunk to run a 500w sub amp back there too.