Ground! The lights are grounded through the light mounts or tractor as you put it. Another way to check to see if they work is run a wire from the light bracket where it would bolt to the mount and touch the other end to a good grounding surface on the tractor. It took me 3 days to get all mine to work. I had to make metal shims to go around the light mount rods so there would be good contact between it and the light fixture. Any little rust or bad contact will keep them from working. They are a royal pain!