Brown = Running and Parking lights
Yellow = Left Turn Signal
Green =  Right Turn Signal
White = Ground

  Not all sunny days are filled with sunshine. Your big trip has been planned for weeks, your packed and ready to go, and today's the day. All that's left is to hook up the boat and go. Right.... All goes well, until you test  the lights. No parking lights, and when you turn the turn signal on, everything flashes. What to do next can mean the difference between minutes or hours of trouble shooting. 
  There are several things to keep in mind before doing any thing rash. The first thing to remember is that the entire light system has one common link, and that is the ground. The lights are usually grounded to the frame of the trailer. The white ground wire, in the trailer light plug, is usually connected at the hitch coupler. This can be a problem on trailers that are bolted together. The often loose and rusty bolts that secure the tongue, frame, light brackets, and fenders are supposed to carry the electrical load to the light system. Due to loose paint, rust, and trailer flexing, it is often these locations where there is excessive resistance to the electrical currents.
  In the example at the beginning of the story, this is most likely the problem. Since all the lights flash, the power from the turn signal is trying to return to the car through the running light system, which is grounded to the car through the light bulbs themselves. On some vehicles, you may even notice that all the lights are flashing on the vehicle as well.
  In some cases, not as obvious as the one above, any one part of the trailer and or the frame could have lost ground and that particular light would not function properly and or flash at the wrong time. The trick to remember is that even though a light will light up, it still could have a bad ground or connection.

  The first step in finding out what's wrong is to see what's right. Check all the light systems and see what is functioning correctly and what is not. This will give you a clue to any common problems similar to the one listed above. Then isolate the cause as a component rather than a single light source. By that I mean that the light, light bracket, fender, frame rail, tongue, coupler, and white wire to the ground connection of your vehicle are considered the ground path component. If every thing else works but one light... the light and it's bracket, are to be suspect and you can ignore the rest. Oh, and by the way, as mentioned before, just because the turn signal works inside a specific light fixture, that doesn't mean the ground is good to that fixture.

========== (Work in progress) ==============

What's to come;

coat hanger as a test tool

fogging  oil and trailer lights

sure fire roadside light bulb test

Liquid tape and silicone tips to seal fixtures that aren't sealed. (only seal the top half of the fixture lens with silicone so that the lens can be removed latter without breaking the lens. Only use "silicone type," for it will peel away from the plastic fixture easily. Test the sealer on the fixture before you do this, if your not sure you have the right sealer.   By sealing the top half of the fixture you will create an air bubble in the fixture that will reduce the amount of water that contacts the bulb and socket.      Liquid tape or less preferable silicone should be put on the back side of the fixture, where the wires enter the fixture.

NOTE: Never completely seal a light fixture. The fixture lasts longer if it's allowed to breath. Drill a very small hole in the bottom of the fixture, if condensation seems to become a problem.