Yeah, you probably need to wait. I have a 1986 BMW 528e (not that it's very similar to your C, but the radio issue seems to be about the same). Because I did not have the code, I entered random combinations. The anti-theft light flashed whenever the car was running (not when it was off, as it usually would). My mechanic showed me where the 5-digit code had been scrawled in pencil on a back page of the owner's manual.
If the wrong code is entered into my radio 3 times in a row, you must wait 3 hours before trying again. Do it wrong again, and you must wait 6 hours, then 12 hours, etc. During the wait period, the radio must be left 'on', which necessitates leaving the key in the ignition.
Hope this helps.