Since mp3 is a proprietary format, the specification is not so simple. This site gives a good overview, I think.
In regular mp3 files, the ID3v1 metadata tag goes at the very end of the file, it is the last 128 bytes. This is actually a bad design. The ID3 system was added as an afterthought in mp3, so I think there was no other way to do this without breaking backward compatibility. This means that if the radio stream is provided as an endless mp3 file, in the normal sense there can be no ID3 tag.
I would chat with people who run a radio station; maybe they put the ID3 tag in a non-standard place?
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