You inadvertantly sprung the theft-deterrent system on your Becker radio, but your model does not have an owner-entered passcode. Unfortunately, you need to get the radio to your MB dealership where they have a special device to reactivate the unit. (Real pain...)
The good news is that the radio is almost certainly undamaged.
1991 560SEL-Euro (145,000 miles)
1989 420SEL (Retired at 325,000 miles)