Detailed instructions on how I installed the P2 in a removable stash tray using industrial Velcro. (Scroll down if you want to use the P2 mounting tray instead.)
To answer everybody's first question: I found the white 5-pin female harness connector behind a large (1" diameter) black-tape-wrapped bundle of wires running vertically on the left side of the parking brake pedal. With determination I was able to find the plug, toward the top of this bundle. It was not concealed by tape, just tucked into a corner behind so many other wires that it took a couple of hours of intense searching by me, a pro hitch installer, and 3 service guys at the dealership. There were 3 decoy plugs within 2" radius of the one I needed. The decoys are either capped or have too many pins. If you find them, look above them. The towing harness plug is obviously right, just not obvious to find. I removed the kick panel but no part of the dash.
I connected the Tacoma jumper harness to the Prodigy P2 harness inside a long piece of heat shrink (my wiring notes are below the photos). I ran the combined wires above all the pedals and across to the right, above the accelerator pedal. I was careful not to rest the wires against the 2 hot metal tubes above the driver's left foot. I used a couple of zip ties along the way.
From there I ran the wires rearward (in the direction of the shifter) behind some carpet beside the accelerator pedal, and into the little stash shelf beside the ignition switch. It sits level within 20 degrees side-to-side, per Tekonsha's spec. (See photo below. Also see gnfreak's install photos of an older controller in this location.)
I notched the back wall of that removable shelf (photo 1) with a hacksaw and also removed a 2" plastic tab on the dash (photo 2) by scoring it with a sharp knife and bending it off. It was easy to modify this soft plastic. The new notch is just wide enough for the rectangular plug that connects to the rear of the P2. This allows the P2 to tuck deep into the tray and stay out of the way of the ignition key.
I applied self-adhesive Velcro tape to the top of the controller and attached it to the top of the shelf cavity. When I'm not using the P2 I can snap out the shelf, remove the controller from the Velcro, detach the wiring harness, and pull the wiring behind the dash so it's out of my way.
That's it. Thanks to Maverick491, psmcn, and others who posted helpful photos and explanations of where to find the wiring connector and how to install the brake controller. Plug for Mann's Hitch, and Toyota of Kirkland, who both helped and charged me nothing.
Square notch cut into removable stash shelf (ignore the smaller notch at the bottom of the larger one, it was unnecessary).
Tab removed from opening at top rear of shelf cavity.
P2, wiring, and modified openings in shelf and dash. The shelf isn't quite snapped in place in this photo to more clearly show how the wiring passes thru the tray notch and the enlarged opening in the dash behind it.
Tekonsha Prodigy P2 trailer brake controller installed in 2011 Tacoma.
MY wiring connections:
(Tacoma jumper harness* -> Prodigy P2 harness)
Black-Red -> Black
Green -- not used
Green-White -> Red
Brown -> White
Red -> Blue
*Factory-supplied with V6 and Class IV towing package. Part No. 82132-0C010
Here's my gallery on picasaweb with more photos: