I have an older Harley with a 2 wire going to brake light now, red and black. I have a new LED brake light with 3 wires coming out of it, red, black and yellow. What are the different wire go to, black to black, red to red, and what is the yellow for? Or can it be fixed to run on the 2 wire system? Thanks

 

It is most likely IMHO that one wire is ground, the other two are tail-light and brake-light. On older bikes ground is achieved through the bike, which as that is now done with a wire indicates how reliable that is! It is easy enough to confirm that using your bike battery and some extra wire, or test wires. I would guess the black is ground, so touch that to the negative terminal, then touch the other two wires in turn.