Well I wired it inside the driver side kick panel and attached the switch and relay to the diagnostics port. Here's the final result, and the switch wound up in the perfect position to be out of the way but not difficult to reach.
Up is Jeep input and Down is for Flat Towing.
If I leave it in "Flat tow" while driving the Jeep my brakes and parking lights work, and the front turn signals flash really fast (as if a bulb were out).
If I leave it set for driving, the brakes will still light through the wiring harness, as will the parking lights. The only missing piece will be the turn signals.
I'll take some more pics when I've added the tow bar.