I have heard of several instances where the 'prewired backup camera' wiring is wired to the camera mount, but not to the clearance light that powers it. The wiring is wrapped around the clearance light wire, but not connected. This appears to be only on some of the most recent builds.
Remove the center clearance light and check for connection there.
Let us know what you find.
Doug and Patience and Shelby the mutt
2015 Crossroads Sunset Trail Reserve ST26RB
2013 Ford F-150 3.5 EcoBoost XLT Supercab 4x4