EDIT: See two posts below for information on a more convenient location for camera connections not requiring work in the overhead of the truck.
Here is how the OHD monitor connector should be wired to drop signal down to Nav unit. For a shorter run there is a connector under the driver's side A-pillar cover that has the same circuits with the same color coding. It is one of two connectors down low right below the curtain airbag mount. The one that can be used is closer to the driver with tape wrapped up it. You won't have to mess wit the headliner that way.
Here's the diagram of how I finally got things to work after finding the broken wire in my tailgate harness:
I'm still not convinced there may be similar circuits in the unused K9 plug or in the K3 plug behind the Nav but forewent the continuity check because I already had everything set up and buttoned up.