It sounds like a short related to the third brake lamp. If you remove the light from the third brake light, do the brake lights and signals work properly?
Next, I attached the wiring schematic below. It was three pages long and I had to cut each picture in half to make them readable. I did overlap them so you can follow from one to the next.
I highlighted the wiring related to the issues. Do me a favor, note that there is one ground for the rear lamp system. They are all combined via the rear lamp junction box.
The ground is located at the left rear body mount of the vehicle. Check the condition of that wire/ground. Make sure corrosion or a poor connector is causing a problem at the body mount.
Let me know what you find or if you have other questions. Also, here is a link you may find helpful:
See pics below.
Images (Click to enlarge)
Tuesday, August 31st, 2021 AT 9:01 PM