[Happy_dan]

I am interested in this as well. Mine is mounted to the steering column and it has the same line to the prop valve.

This is my theory, the line to prop valve moves the lever on the controller when you step on the brakes. That creates an electrical signal that goes back to the plug at the rear. That activates the electric brakes on a trailer. This is just my theory and if correct would indicate you need some kind of power back to the plug. Mine has wiring that runs along the frame rail to the rear and taps in at the back of the harness. I can get pictures of all of this if you need it.