1997 Chevy Silverado Brake Light Circuit

How do I find the brake light circuit at the rear of the truck? I have a third brakelight above the rear cab window, but I want to add a third brake light to a topper at the rear above the tailgate. This topper was made before the third brakelight requirement and I have one I am installing. The wires that go to the right and left brake light recepticals are the combo brake/turn signal circuits like on trailer wiring. I do not have a dedicated brake light that I can pull from in the receptical area. If I use one of these combo circuits, the third brake light will blink when I turn on that turn signal. Is there a brake only circuit back at the tailgate, and how do I identify it easily, or do I have to go forward to the existing third brake light and run a wire to the rear of the truck?