I recently did a test-of-concept install of my P3 brake controller, on my 2011 Honda Pilot.
-Must not be under the dash where my knees and legs hit it.
-Must be where I can see it while driving and reach the manual lever, without taking my eyes off the road.
-No holes in the dash (wife factor)
Here's what we have.
I noticed an end piece that had a place for a screwdriver to be inserted, to pop it off.
Honda gave us a hole just big enough for the controller cable connector to fit through. There is a a plug under the corner of the dash that connects to the factory prewire (2009-2011 Pilots have it as a standard feature)
I used a Simpson LSTA 18" strap to fabricate a mounting platform. It's only attachment is to that end plastic piece, with two small bolts/nuts (It's not in the dash itself, cannot be seen when driving, so it passed the wife inspection.)
The other end rests on the dash curvature, with a piece of felt underneath.
I used a peice of shirt cardboard for a template, then bent the metal strap, and cut a rectangular hole for the cable connector.
The controller bracket is mounted with two bolts. The left hand hole in the metal strap is a slot, so I could rotate the controller bracket so the controller is lined up with the centerline of the car front-to-back (Per the Tekonsha P3 installation instructions).
I test drove it: no vibration, no front-to-back movement, no rotation, even with hard braking.
When I operated the manual lever underneath, I tend to move the right hand side of the bracket, so a small piece of velcro may be in
order. (At least it won't be screws or bolts in the dash).