I'm considering doing the RPA retrofit in my '04 4.4i as well.
The only info I could find on it is this install (uses parts from an E39): http://www.bimmerboard.com/forums/posts/179456

Is there any wiring diagram of the X5 posted somewhere so I can try to find where the wires I need to tap into are? And/or if anyone has already done this install in an X5, any info would be much appreciated!!