I recently got an email from one of the frequent posters here that you guys were looking for some help on doing the sirius retrofit that I just did on my E38.
This retrofit is easy, and would be easier in the E39 than the E38 since the radio is right up front is in plain view in the trunk area
The radio is on the left side of this picture and a piece of cake to get to.
I am going to work on making a details documentation on making the the audio input adapter cable which is the only adapter piece that I was unable to find off the shelf from BMW.
If you guys want to do some investigating, you could call a dealer and have him dig around for the old style 10 pin to new style 12 pin radio audio input plug.
Since the radio is so easy access in the E39, the retrofit would take you guys about an hour max
http://www.bmwnav.com/sirius.htm
Martin