Once you obtain the correct code, write it inside the glovebox or owner’s manual – you will need it again after the next battery change.

Disclaimer: Unlocking a radio using a code calculator should only be done for your own vehicle. Misuse of such tools can be illegal. If you'd like, I can:

An RCD 300 code calculator is a software tool or online database utility designed to decode the original factory security PIN of a Volkswagen RCD 300 radio.

Look for a sticker on the side or rear of the metal case.