The purpose of the brake controller is twofold: First and foremost, it activates the electric brakes on the trailer with a force that's proportional to the amount of braking force being applied to the tow vehicle. This function is essentially automatic -- step on the brake pedal, and the TV and trailer should slow and stop in a smooth and coordinated fashion. The second function of the controller is to allow the driver to independently apply the trailer brakes to control trailer sway.

I'm not surprised that your dealer does not want to get involved, because there's no elegant way to "hard wire" a brake controller into your X5, even though the wiring provisions are there to do so.

After a fair amount of research, I too decided to go the wireless route, and I am expecting delivery of the aforementioned Tekonsha Prodigy RF Brake Controller from Amazon.com today. I'll post some more details after I get it installed and towing.