There is an old post on this but I had to do some digging so am posting this for other RAM owners.
My truck is a 2007 Ram 2500 with the 6’4" bed. After some research I learned that the best way to connect the GFC third brake light is to use the CHMSL (Center High Mounted Stop Light) wire that comes out of the wire harness on the Drivers side, behind the spare. On my truck this is a white wire with a tan stripe and it’s sticking out of the harness, tied back and heat shrinked. Connect the red wire from your third brake light to the white CHMSL and ground the black and it should light only when you brake. I solder and heat shrink my connections and put a sealed connector on the brake light leads. Cheers.
