Have you checked the bulbs?

What happens when you connect a battery directly to the trailer connector's ground and individual pins?

Laszlo

Corrosion is a likely cause. If the fixtures weren't water proof....ed then the connections and or the sockets are corroded. If the wires in the connections are now black......corrosion. 

Check the lamps....aka bulbs.... first thing. Then check connections. Clean out any corrosion. Use shrink wrap on splices.  I use twist nuts on my fixture connections because they can be quickly worked on. I use an electrolitic grease. See West Marine.  This grease seals but allows electrical connectivity. Regular grease is an insulator. I coat the connections, once made, with the electrolitic grease. I coat the rubber gasket/ring on the light lense. That keeps the water out of the light and the sockets. The connections get the grease after connections.

Usually the grounds will be the first thing to go. Whether it is a wire connection or a connection between fixture and frame.

I operate mostly in saltwater. The grease keeps my lights working for a year at least with regular year round use.    

A bad ground attacks the turn and brake lights first. 

Check the pin connections in your connector.

I am unfamiliar with your specific trailer. However, I recently had a new kayak trailer that lost all tail and side marker lights. I was able to jumper wire the installation and it worked. The problem turned out to be "inside wiring". The structure is square tube steel with the wires run internally in the tubes. The tail light wire(s) was spliced at the side marker lights inside the tube. Adjusting the pig tail at the hitch to the truck hitch configutation pulled the wires from the splices at the side marker lights. I had the same problem you describe. No side marker or tail lights. Check those connections. Some disassembly required.

   Too new for corrosion, most likeley a fuseturn on lights and flashers, hook a test light to ground on plug from truck there should be voltage (2 flashing)on all the other pins if not you've got a blown fuse.sometimes it can be hard to find which one there can be multiple fusee boxes and  they can be mislabeled. If you've  got good power from truck use some jumper wires and a couple of pieces of wire attach groung to trailer grund pin and toch the positveto each of the pins they should all light something. At that point you'll know if its the truck or trailer

1. check all of your grounds. If your trailer is aluminum, use a green pad to scuff up the ground points until they are shiney.

2. Spend a little bit extra and get water proof heat shrink crimp splices.

3. When your lights break, replace with water proof lights. not submersible.

Following my above advice, I haven't worked on my trailer lights in a few years. Good luck, JRC.

   Thanks all ! Will keep working on it and report back.

    I think it's a good idea to unplug your trailer wiring from your vehicle when backing the trailer into water.   Assuming you left it connected this could be part of the cause for the problem. 

Just a thought, Jack

Having sailed Hobies for years in saltwater I found that the LED lights are the only way to go.  Submersible just do not last.  Ditto on using the sealed head shrink tubing on all splices.  Ditto on the grease too.  To help combat the grounding issue I have taken the extra effort to use an actual wire for the ground instead of the frame on my old Trailex trailer.  You may have to get new lights that allow an easy and robust ground connection.

The little extra effort now saves alot of effort later.

Nick