Hello Dave,
I'm going to install my brake contoller this weekend, so I have just finished all the research myself. I recommend the Tekonsha Prodigy controller, if you haven't already purchased one. I used the Prodigy on my last truck and it was truly awesome!
I've done these installs before and they are an easy project for any do-it-yourselfer using the following instructions:
There are two excellent write up's for installing the controller located on the home page of this site -
http://www.bmwmotorsports.org/pdf/BM...er%20Brake.pdf
http://www.bmwmotorsports.org/pdf/X5...stallation.pdf
The wiring is straight forward (to anyone who has done this type of thing before), and there is only one aspect of the installation unique to the X5 - picking off the 12V brake signal. That connection is described from BMW here:
"A connection pin for retrofitting (electric brake) is provided on
plug X393S. If an electrical brake is desired for the trailer, it
can be retrofitted. For the latest list of recommended brake
controllers contact BMW Service Hotline. The signal pick-off for
brake (the input to the brake controller) may only take place on
plug X12, Pin 10. This plug, X12, is to the Light Control Module
(LCM), the wire color is black and yellow. The male pin (BMW
P/N 61 13 1 376 195) should be used at the mating plug of
X393S. Be sure to observe this information."
In addition to your brake controller kit, the BMW OEM tow kit, and misc wires and connectors (that any installer would already have), you only need two additional pieces of hardware: 1) a BMW male pin P/N 61 13 1 376 195 2) a 20A-30A resettable circuit breaker - available at any stereo and most auto parts shops.
So here is the readers digest version of the install:
1) pick your controller location - left of sterring wheel or inside ash tray
2) black and white wires (power and ground) to the battery
3) blue wire to the rear trailer connector
4) red brake signal wire to the LCM (light control module behind the glove box)
5) Done!
Simple, no?
Good luck, let us know how it goes!