3rd brake light on camper shell wiring

[Thequickster]

Hello I have a 2021 Silverado Custom Trail Boss. I recently purchased a rough country camper shell used and am trying to wire the 3rd brake light. My first attempt was red from the 3rd light to green on the factory tailight wiring, and black from 3rd brake light to red on factory wiring. This causes the brake light to work but also the 3rd tailight to come on with the blinker , does anyone know what wires i should connect to. And no I dont have a test light at my disposal right now. But will buy one if no one knows the answer. Thanks

 

[PistolsFiring]

https://www.etrailer.com/question-718507.html

Maybe something like this. FYI I have no experience with this.

 

[Thequickster]

This is the instructions with the shell on rcs website, so it must be able to be done with the taillight wiring

 

[Thequickster]

Just got off the phone with Rough Country they told me they have no idea.