The left blinker and break light on my trailer are not working. I know that the yellow wire operates these and it is "hot" coming from the truck and the trailer when I tested them. When I connect the yellow trailer wire to the yellow light wire it goes "cold" and does not work, same with the breaks. I sanded down a small area to ensure the ground is good. I also checked the fuses in my truck for the left blinker and they are good. Any idea why the connection would go bad when connecting the two wires? thanks