Just picked up GFC early Jan and finally got around to wiring the 3rd brake light. In case this is helpful for anyone else.
- Remove tailight: this video shows which bolts and how to pull it off: New Tail lights for my New Tundra! 2014-2021 Tundras! - YouTube
The bottom connection on the taillight is the brake lights. middle is reverse, top is turn signals.
- run wire through small hole in bed
- Use wire tap connectors to tap red to the blue wire (brake) and black to the white wire (ground)
- put it all back together