I would agree that it connects behind the taillight but I'm not sure about going through that rubber plug. Do you need to remove the side panels, and the rear floor panel to run the wire?
There are clips and plugs attached to the harness, I wonder if the harness drops down from the taillight clipping to the metal folds along the exterior. The plugs fit into the cross member between the mufflers and route back up to the other taillight. The four pin trailer plug in nicely outside of the vehicle at the trailer hitch rather than running it out the tailgate over the bumper.
I'm not saying I'm right (which is why I'm here searching) but this appears to be what it is set up for.
By the way, I don't see the need to remove the bumper as earlier posted. Just like the installation of the hitch it would fit around. There is a video available on eTrailer how to install the hitch too.
I hope someone can comment. Thanks!