I should update this post by saying that my install ended up going without a 'hitch' (sorry, couldn't resist...), and I've towed my race car successfully since with no major issues. I bought the prodigy and used 2 installation articles to work out my install job, pulling info from both to come up with my own install plan:
http://www.bmwmotorsports.org/pdf/e5...er%20Brake.pdf
http://www.unofficialbmw.com/e53/x5_...er_install.pdf
In my opinion, to get the cleanest install take the time to go through both articles completely before beginning. I elected to mount the Prodigy under the driver side lower dash trim, so that I could remove it when not in use.
You take the brake light signal from the LCM, which is located behind the passenger footwell panel. Note that you will see 3 plugs connected to the front of the LCM. You are going to be splicing from pin 10 of the TOP plug. Your plug may not have as many wires connected to it as the wiring diagram graphic that is posted somewhere up here. Don't worry about that. You will still splice from pin 10. The pin numbers are visible on the front of the plug. There is no need to unplug this connector. Although room is tight in there, you will be able to move the plug wiring around enough to where you can get to the brake light wire. If you have any questions you are welcome to PM me here and I'll do my best to help.
BTW, the Prodigy is quite easy to setup and use. I was quite impressed with it. Just make sure you take the time and follow the instructions on how to set the braking power before you start towing. That is very important.