When I bought my 1983 trailer in 2009, all of the connections were corroded so badly that I had to replace the tail lights and most of the running lights.
Open up the fixtures, strip the wires to clean copper and test vs. a good ground.
Start with grounds to the tail lamps, then move on to stop/turn using a jumper battery and clip leads at the 7pin connector.
Expect nothing is intact. Write down the wires that are OK. Eventually, you'll find only 1 or 2 broken wires, but all of the connections need cleaning or replacement.
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck
Sitting in The Cheap Seats.
And proud of it!