Under the seat, the 14 pin gray plug. It is the green wire with the red stripe, think it has some white dots on it too. You can wrap a bungee cord or something around the brake lever or step on he brake lever and unplug the pin connector, when you have the right one the brake light will go out. Then double check the wire with a meter. But it is the green wire with red stripe.
Just did it this last weekend. I found it under the seat on the righthand side tucked with another harness between the fender and the frame member. It had a white connector. Unplug it and turn the ignition on. If the tail lights dont operate its the right harness. Its a green wire. There is a larger one at the one end. Thats not it. It's another green wire i beleve thats 3rd down on the one side of the conector. I used a test light to determine the right wire. Hope this helps.