Sorry for the late answer, I just found this.
My idea is to use one of the "thru firewall" wires and go straight to the fuse box or, even better, the battery. Ford has provided a small bundle of wires that dead-end on each side of the firewall just for things like this. You don't want to cut into the factory harness anyway and risk warranty issues down the road.
You can find the under hood end of the bundle just behind the fuse box and below the master cylinder area, The "inside" end of the wires are tied up above the brake pedal. Use an ohm meter to make sure you have found each end of the same wire and go!!!