Photos in the thread linked below might be helpful. I found this using the search function shown in the solid blue line at the top of the page. There are two blanks in the drop down tab. I find that the Google custom search works best. In this thread one of the forum members offers a pdf wiring diagram of a 77 model. You might click on the user name and send him a message asking for a copy.
Check ground at each light. Each light is grounded to the trailer skin via the screws or rivets holding the fixture on. Cleaning/removing corrosion will help