radio on 1991 Mercedes-Benz 500SL

How to reset radio code. The words wait show up

Asked by for the 1991 Mercedes-Benz 500SL
If you dont have the correct code dont try it.Get it from the dealer. You have to wait 24 hrs if you try 3 times and fail. Thats why it is reading wait.
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All of the sudden, my radio wont play and is asking for a security code, I dont know it. Where do I find the security code? Is there a website I can find it on.