Diagnose the truck and trailer separately.
Diagnose the truck connector:
1) Go get a multimeter. You cannot test without one.
2) On the truck side of the connection find the 4 wire hookup
3) Looking at the connector you will see one position completely exposed (male) and 3 positions that are encased in rubber (female). With the multimeter set on DC, put the RED side of the probes in the female connector closest to the male connector
4) Put the black multimeter probe on the mail connector.
5) Press the UNLOCK button on your key. I am assuming you are troubleshooting alone and this is the easiest way to get momentary accessory lighting. If the multimeter shows close to 12 volts then that connection is good. If not, you have an electrical problem or you didn't read the instructions for your multimeter.
6) Assuming the accessory lighting is good, move the RED probe to the next female connector on the plug. This will be the middle female hole.
7) Turn on your LEFT turn signal. You should see the voltage spike and drop on the multimeter with each on/off of the turn signal.
8) Assuming the LEFT turn signal is good, move the RED probe to the next female connector on the plug. This will be the last female hole.
9) Turn on your RIGHT turn signal. You should see the voltage spike and drop on the multimeter with each on/off of the turn signal.
10) All good? Then your truck is fine. If not, its time to check the trailer.
Diagnose the trailer:
1) Get a set of jumper cables and to small pieces of wire. I use a 1 inch piece of coat hanger but use whatever you have. CAUTION: You are going to be connecting the jumper cables or other wire to your battery. DO NOT let the other end of the wires touch!
2) With the front of the truck up to the trailer 's hitch, open the hood
3) Connect one end of the jumper cables or some other long wire to the + and - terminals. You need a known good 12 volt source to test the trailer lights.
4) On the other end, use the jumper cable clamps to hold the small wires. You need these small wires to touch the points on the trailer harness. Remember, DO NOT let these touch!
5) The BLACK or GROUND side of the wire from the BLACK or NEGATIVE side of the battery needs to go in the only female connector on the harness
6) The RED or POSITIVE side of the wire from the RED or POSITIVE side of the battery needs to go to the first make connector on the harness, right next to the female connector.
7) As soon as the positive side touches the male connector the running lights on the trailer should light up, including the tail/brake lights. The tail lights will NOT be bright as if the blakes are applied.
8) Move the RED or POSITIVE connection to the next male connector. This is the LEFT turn signal/brake light. That light and ONLY that light should come on.
9) Move the RED or POSITIVE connection to the next male connector. This is the RIGHT turn signal/brake light. That light and ONLY that light should come on.
10) All good? Then you have some other issue. Take it to the dealer.
One thing you can check is ensure there is no water anywhere in the truck or trailer side of the connectors. The water could short out the wires.