This is going to take a few tries. If you look at the attached pic below, I circled the wires that are near the high mount brake light which will have constant power when the headlamps are turned on.
Let me know if the 3rd brake light comes on with both the headlamps and the parking lamps. Next, for that light to come on with the headlights, there has to be a short to power in one of the tail light/parking lights wiring. Follow the red/green wire back from the light itself and see if you can find anywhere it is damaged and shorted. The red/green wire to the third brake light isn't too long. It will tie into the brake light wiring in the rear of the vehicle.
Let me know what you find.
Thursday, November 19th, 2020 AT 5:56 PM