Looked in the factory service manual and if the trailer harness uses the factory supplied connection point, the wiring goes to the drivers side fuse box under the dash (near the parking brake pedal). You'll have to check if there's a fuse specifically indicated for it.

You might check for power at the trailer connector with a volt meter - you can look up a 4 wire trailer harness to know which one is which - you should have ground, 12v+, parking lights, brake lights.

The trailer harness wiring should lead into the body through a hole near the spare tire, you could check if the wiring insulation is in good condition. The factory connector for the harness is under the threshold of the tail gate. I would check for power there too. If there is power, the harness going to the trailer is probably bad. Here's a link that shows the factory connector:

Good luck!

 

In the copy of the FSM I have the 2 "trailer" wiring hits I got were for the under hood fuse 47 - 20 amp, and passenger foot well fuse 10 - 15 amp. But they fuse a lot of other functions (wouldn't let me cut and paste) so you should have lots of things not working on your car.

For 4 wire the colors are
White - ground
Yellow - Left turn lamp +
Green - Right turn lamp +
Brown - Parking lights +

If none of the functions are working I'd suspect a bad trailer wiring ground, or the converter box fried.

Not knowing exactly how you tested, I'd suggest,
Doing a continuity check from the connector white terminal to any solid metal non painted part of the body. You should see very low resistance. If you see high resistance the ground is bad. You need to chase down chassis ground G602. I didn't look up where it came from.

If the ground was good, check for voltage between white, and each of the colored wires, while that light function is blinking (or parking brakes are on). If you get no voltage, then I would suspect the converter box. You could verify that by disconnecting the converter box, and test the car side harness in the cargo area that LeftOverParts linked to in the Uhaul video. I'd have to look up further which wires are each function. And of course as he said check every piece of the wiring harness for nicks and cuts.