Why is my taillight out?

fuses do not cause burnt bulbs and finding the fuse will not fix the problem
replacing the bulb will

Change the bulb. When you press the pedal the brake switch changes the circuit to the 21w brighter circuit for the brake light.

use a test light unplug wiring harness beside frame rail and test to see if you have power at plug most the time it is a bad ground going up to taillight

The only thing left is the wires and the bulb socket. The problem has to be there.

You can test any relay, if you know how they work

On side is the primary coil to energize it

The other is the higher current contacts.

There should be a diagram on top of it

Replace the light assembly.

Yes. But do you have seperate yellow turn signal lights in the rear or do the turn signals work independant of the brake lights? If yes then you need to wire in a taillight converter. It takes your seperate left & right turn signals combines them with your brake lights on 2 wires instead of 3 leaving you with your normal 4 wires in total instead of 5. 1 Left turn & brake light. 1 Right turn & brake. 1 Taillight. 1 Ground. = 4 wires in total. If you dont have seperate rear turn signals just find left turn wire in taillight assembly, Right turn wire & Taillight & a good ground & you are done. On trailer plug yellow is left, green is right ,brown is tailight & white is ground.

you got a bad ground on the taillight circuit

You have a burnt out tail light replace it.

Check the electrical grounds for the taillights. In fact, get a diagram and check all the easily accessible grounds. Many of these are in the engine bay.

Dan