Originally Posted by fubar 1958
I have found with most trailer lights the best way to wire them is to run a separate ground wire from each light to the ground wire on the trailer plug. That way you have no issues with the grounds at the trailer lights themselves, which 75% of the time causes the issues you are having!
To add to what fubar said:
I have had issues kind of like this as well. Even had this same trailer with the same issue on the rear light. I ended up using a hard wire brush to scrape off the paint on the trailer where it was grounded with the ground wire, and then I did the same for each light and grounded them directly to the frame. Some trailers will use the bolt on the back of the light as the ground, and it doesn't always make good contact. Try running a wire with some crimp on terminals from that bolt, directly to the frame. Self-tappers work great for connecting them to the frame. I bet this is the problem with your tail light.