I've pulled a 31 foot 8,300 pound GVWR Airstream travel trailer for over 23,000 miles using a "Warner" foot brake controller. It taps 12 volts off of the firewall positive jumper bolt and the wire goes immediately to an automatically resetting breaker switch. It then follows a wiring harness through the firewall to the brake pedal where the controller mounts, then from the controller it goes along the rocker to the rear where it is spliced into the 7 pin connector wire harness. The brake controller wiring DOES NOT interface with any of the X5’s electronics.

It only adds about 1 inch to the top of the brake pedal which I readily acclimated to. MOST IMPORTANTLY, if the controller is mounted with it's hinge at the bottom of the brake pedal it can be "toed" at the top to apply the trailer brakes independently of the tow vehicle brakes. I ALWAYS "toe" the trailer brakes first so as to be in the habit to "straighten" the assembly in the event of a control problem AND to minimize tow vehicle brake wear. I currently have over 64,000 miles on my 2001 X5 and my original brake pads are only about 1/3 worn.

In the event of an emergency stop all I have to do is hit the brake pedal and the whole assembly’s brakes are applied to the max.

I’ve made numerous posts on my set-up and towing, most on the http://bimmer.roadfly.com/bmw/forums/e53/ website where you can do a search on “withidl” to find them.