You should have got a radio code or stereo security code card when you first purchased your Mercedes. Look in the owner's hand book. The dealer that originally sold the car may have the number on file.
the code may be in the original owners manual. m-b dealer can give you the code from the vin number if it is the original radio that came from your car. mine is a 1998 model and after entering the code i pushed SC (scan ) button it took the code and still working... hope this helps