Re: I'm stumped.......
Grounds on trailers, particularly boat trailers, are always a problem. I prefer not to use the chassis of the trailer as a ground, as corrosion seems always to cause problems with the connections to the metal. Particularly on electric trailer brakes, I like to wire in the grounds without using the trailer chassis at all.
That said, I have seen numerous cases where grounds were bad, and one light or the other would work, but not both. To test this out, make yourself a long jumper wire, the length of your trailer, with alligator clips on both ends. It may seem kind of elementary, but remember that you need only +12 volts and ground at each light to make them work.
Clip one end of the jumper wire to the tow vehicle ground (perhaps even to the negative terminal on the vehicle battery), and the other end to the ground of the problem light fixture. Then test the lights and see what works, and what does not. Based on the results, you may be able to come to some conclusions as to where the problem is located.
I also find that the standard, rather simple 12 volt test light is highly useful for trying out trailer light circuits.
Good Luck!
skinorth