I don't know why the Prodigy (etc) won't work, either. But you are right, if the brake light switch is in the cold side of the brake light wiring (quite possible), that would do it. A wiring diagram for the X5 is needed.
If you can't figure it out, there is another good solution. Consider a Jordan Ultima 2020 controller. I've had one since 2001, and I love it. It has a three-wire connection (rather than four) because it does not require a connection to the brake lights. For my original setup, that was necessary.
See a picture and read about it at
http://jordanbrake.com/ You can see a U-shaped bracket in the lower left corner of the picture. This bracket is clipped onto the brake pedal arm, so when you depress the pedal, it pulls the cable which activates the brakes in direct proportion to how hard you push the pedal. About as simple and foolproof as you can get.
RockyMtnRay mentions this controller in his brake controller tutorial in the TM Reference Library.
Just a thought.
Bill