A few questions to see if you have a radio issue or a larger issue:
Radio issues only? In other words are there any other issues that are random? Turn signals, four way flashers, etc.

• When rotating the key, do they work then stop?
  What is the voltage at the Battery (in the trunk) vs the battery voltage at the terminals under the hood at engine off?
  What is the voltage in theses two locations when the engine is idling?

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green