Sounds like an issue on the tow vehicle since the light activates like it should with the brake lights. That means the trailer wiring is correct but the signal is just not present on the tow vehicle like it needs to be. It almost has to be a fuse on the vehicle too since the corresponding brake light circuit is also activating like it should so that would be what I would check first.
I attached a help article with more troubleshooting info on trailer wiring for you to check out as well. view full answer...
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