If you have the books for the car look through them completely. I found the radio code scribbled inside the last page of my 400e owners manual. If your radio is displaying - - - - (four dashes) its toast. As mentioned, the dealer can look up the code using the VIN if the radio is original. If not, then yes, they need the radio s/n

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88 300e
89 300e
93 400e
93 500e
77 308 GTB