If it's only the radio there is still a chance it could be something else but likely you are going to need some wiring diagrams and do some diagnostics (maybe take off the steering wheel) to check further. Maybe there is some common problem that someone else can chime in on but I don't know of one.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)