However, a word of caution as of 2025: Many links on Page 1 are now 10+ years old. While the thread still exists, some rapidshare or megaupload links are dead. But the knowledge on Page 1 is immortal.
A code generator is a small software (often written in Java, Python, or C++) that performs this calculation instantly.
GM0301T2345678 or BP1234Y5678901
: The first place to check is your vehicle's owner's manual or the documentation that came with the radio. Sometimes, the code is provided there.