Or you could wire it up under the dash, to the power bar that is already there.
Saves trying to go through the "firewall", especially if none of the accessories are headed in that direction.
See that bundle of wires (white/striped) that are to the right? I think that are already passed through the firewall, and if you look under the instrument panel, I believe you'll find the other ends there.
And if you take up the centre of the floor board, some are down there. best thing to do is get a test light or continuity tester, to check... if the size of the wire is the correct gauge, it might save you a lot of time!
Garmin also makes a gadget like that called the PowerSwitch. similar to SPOD, etc, but smaller, and smarter (and less $).