Isn't the radio on the MOST bus in all E90s??
The bus is a fiber optic loop and if you break it half the displays in the car go away.
My Logic 7 certainly is on the loop and I experienced major problems when installing the accesory iPod adaptor because the BMW (actually Alpine) instruction sheet showed an incorrect connection for the fiber optic interface.
The radio programming also may be interfaced with the main car programming.
You may be able to tie the fiber optics together with the radio out of the system, but these cars are so complex I would hate to predict what would happen.
You better check into these things before you start. This isn't your father's Oldsmobile.