blown bulb due to short circuit, faulty ground, or broken wire
Fuse is blown light connector is unplugged ground wire on trailer disconnected
Yes.
Check the fuse first.
Yes, just like you would anywhere else.
check fuses under the hood
The brake lights in your 1983 Nissan pickup truck are controlled by the brake light relay switch. When the brake light relay switch malfunctions the brake lights will stop working.
Check the radio/Acc fuse...
I would first check the grounding between the truck and the trailer. If the ground is good then bulbs would be the next thing to check.
Make sure you have a good ground
A wire could be broken, a relay could be defective, it could be wired into the wrong relay, wires may have become disconnected, the plug itself may be corroded.
Trailer lights are sold at many stores nationwide. You can find trailer lights at home improvement stores. You can also find trailer lights at automobile repair stores.
I would check the running lights for one with a short. I had the same sort of thing with my trailer and I spent way to much time troubleshooting the brake and signal wires. Turns out a running light bulb connector was grounding out on the trailer and messing it all up.