Sounds very likely to be the radio tuner module. You could elliminate the CD changer by simply connecting the Sirius tuner directly to the harness without looping it through the changer. In most cases the symptoms you've seen have been caused by an out of rev radio tuner. I've seen a few out dated NAV players but I'd be very surprised if that's the case on a 03 model.
What build date is your 03 ? If you can get access to a "new" tuner model from an 04 X5 you could just swap it for testing. I've seen dealers selling "old" stock which didn't have the Sirius fw in them.

- Jan

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