wiring diagram for third brake light?

Hello,

This sounds like the brake light control module is out. To be sure here is a guide to check the outputs to the brakes lights and the brake light wiring diagrams so you can see how it works with the module location (right side of dash) so you can check the electrical connector for corrosion:

https://www.2carpros.com/articles/how-to-check-wiring

Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.