I typed this earlier so it is a copy and paste answer, but is the complete answer. If no registration document, you need a code company.
If it was an OEM radio that came with the vehicle when new, you can take your registration document to a local dealership and they will look up the code for you.
If it is an aftermarket addition, you will need to pull out the radio out and find the serial number on it. It is on a paper ticket, back or side. Then contact one of the radio code websites who will supply the code for a small fee based on the serial number.
https://www.google.com/search?q=car+radio+codes
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