I don't usually post if I have nothing to offer, however I also have a similar radio issue. I have an 04 ZHP that has occasional dead radio syndrome. Without warning the radio simply won't turn on. It also seems to kill my battery now and then. One time it stuck in the ON mode and the volume was stuck at one level. I have yet to duplicate that anomaly. It gives odd readings when the tech's computer is diagnosing. So far the nav, radio module and some relay are in question.
I have it in the hands of a good BMW guy who is checking it out. I will post any favorable news.