Blinkers do not work, fuses all good

There is a tail lights fuse for the truck, but not for the trailer.
First, check that your left tail light on the truck is working, if not fix the problem (new bulb, bad contact, fuse).
Then the obvious suggestions regarding the trailer are: Check the tail light portion of the bulb to be sure it is not blown. If good (best to put in new one to be sure), then check the wiring from the front of the trailer all of the way to the non working bulb, including every connection and at any place it is secured to the trailer frame. If all is good with the trailer circuit and bulb, then check the truck's trailer connector, including any adapters. If by chance the truck is wired with a flat 4 wire harness the problem may be solved (but usually the blinkers are the problem). Modern trucks require a 5 wire harness which self adapts to the standard 4 wire trailer circuit. Check for dc power (12 volts) from the truck's female connector. One wire will be ground, others will be taillights (turn on headlights), brake lights (have someone press brake pedal), and blinkers (turn on switch and blinkers).

iguess you tryed removing this OEM PLUG TO SEE IF THINGS RETURN TO NORMAL?or you can just diconnect power supply to plug to see if there is problem,next use test light when connected with power to plug and see if tester will blow fuse ,it allmost sounds like there is a direct short to ground look over wires carefully look for problems and the plug .hope you get it good luck.

Check to see if trailer wiring was wired correctly, I've seen similar issues where incorrect trailer wiring has damaged the vehicle wiring and cause electrical issues. Lets hope this hasn't happened. Unplug the trailer and test all circuits for functionality.

then if theres power to harness at back of truck its the harness on trailer or wireing on trailer

First check turn signal fuse. If it is good most likely the turn signal flasher needs to be replaced.

It's a wiring problem and in most cases a grounding problem. Check the wiring with a multimeter. See that you have proper current on the hot side and also check ALL of the ground wires to see that they are really grounded. They sometimes get loose an or corroded and do not ground.

Check the fuses in the fuse panel area. Check to see that the bulbs are good. Good Luck, Jim

check you bulb and there sockets its possible that you burned out one of the two

Your running lights/marker lights and tail lights are on the same circuit. I all thetruck lights work, the problem is in the plug or plug harness.

I take it the blinkers and brake lights work ok when not hooked up to the trailer? If yes, then the wiring to the tow hitch is not hooked up correctly. How many wires to trailer pig tail connection? U can have up to 7 depending on what you tow. Check to make sure these wires are hooked up from the truck to tow plug correctly: 7 wired---
brown-Rt turn and brake light, yellow-backup, white-ground, blue-electric brake, green-tail running lights, black-battery charger, red-left turn signal and brake light. Let me know if u have something other than 7 wire, I'll get the wiring color and source info for u.