Sounds like you have to enter the security code for the radio if your lucky it may be printed on the back of the radio if your really lucky you already have the security code otherwise you might have to go visit Mr BMW
Theres a couple of little tabs on either side of the radio with a funny shaped screw underneath (I can't remember for sure I think it is triangular an allen key should be able to undo them) unscrew and pull the radio out
Bavaria Rds Radio provides 520i battery storage, once lost all its systems work, in addition to charging the radio. The perfect radio station, the storage of the. When you power on, turn on, the word "code " radio button then it appear instantly. Radio does not seem to apply to any power.