All of the specific adapter references have assumed a Cequent (which includes Tekonsha
brands) brake controller. If it's a different brand of controller you will need a different connector on the controller end, but it would still help to know which other Toyota models and years use the same one.
We found different adapter harnesses from eTrailer:
Originally Posted by Vermilye
If you have the Tekonsha Prodigy P3 Trailer Brake Controller, this cable adapter
is the one I used for my 2016 Tacoma Off Road.
These are both by Tekonsha and clearly different, as apparent in the photos of the vehicle end and confirmed by the drawings in the associated installation manuals. I might ask eTrailer what is going on, since the 3040 model obviously fits the 2016 Tacoma (and older Sequoias and Tundras judging from the photo in the instructions) if that's the one Jon meant to link, but eTrailer says the 3017 fits current Tacomas (and Tundras).
The Tekonsha site - a better reference for their specific products - shows the 3017-P
as the correct adapter for their controllers in any variation of a 2016 Tacoma (and only
current Tacomas and 2015-up Tundras). Tekonsha also says that the 3040-P
is for the previous style of Tacoma (and a wide variety of other Toyota/Lexus models and years).
Jon, are you sure you used the 3040-P, and not the 3017-P?
Edit: See Jon's response below - he did use the 3017-P, not the 3040-P
I would buy the one that looks like it matches my truck's connector. If unsure, they're fifteen bucks each... buy both.