As I said above, the official Ford trailer brake controller does not use the white and violet-white wire, only the yellow and blue wires and a connection to the high mount stop light. Please re-read my reply above.
In the Sync settings menu, sound menu, scroll down to sound mode and change it from stereo to surround. It probably got changed to a default of stereo. This setting causes the rear speakers to function as you described.
They are for the wiring to an optional trailer brake controller. The white is ground and the violet-white is a brake pedal switch signal. The Ford official brake controller doesn't use the violet-white wire for the brake pedal signal, it instead uses the high mount stop light wiring for the...
I am not saying that it is safe to use a strap attached to the ball, but I was pointing out that there is an actual hitch that the bumper attaches to, so the ball is not only clamped to the bumper (by the ball's threaded nut), but also to the built-in hitch. This is how it comes from the...
The owner's manual states that the rear bumper ball hitch is rated at 3500 lbs. towing capacity, not several hundred lbs. There is an actual hitch mounted under the bumper that is quite substantial.
It needs a ball with a very long threaded portion to mount on the bumper mount, however.
If you understand what that poster is saying and apply his fix, you should have your fix for your opening problem. Is it clear now? It was clear to me when I read through these posts. Did you try it?