Fuse locations 2004 f350, brakes not working

There is a 30 amp fuse listed for your truck. Should be position 101 and it is a large fuse not one of the Mini fuses. Look under the hood most likely on the drivers side for the power distribution fuse box. It is rather long. A bad ground could possibly cause the brakes not to work. Do the running lights work on the trailer? Check the brake wiring at or on the axles. Possible the wire insulation may have worn away and bare wire to metal grounding.

Should be a fuse box diagram in your owners manual, showing each fuse and function.

I would use a multi-meter to test, starting at truck connection. A simple 12V light tester should show current, while helper is applying brakes. If good, I'd move on to junction box, near pin box, that cord goes into, to check for brake wire power.

Jerry

What type of brake controller do you have?
Does the brake controller light up when operated manually?
Some after-market have an in-line fuse. Trace the wire from controller to truck connection.
Check the wire from the brake light switch to the controller.
Integrated brake controller should have fuse in trucks fuse block. 2004 maybe the one left of steering column under the dash.
Do not confuse a 30 amp trailer power feed with brake.

Assuming you have the FACTORY trailer tow wiring, see page 194-199 of the 2004 F350 Owner's Manual. Position 101 is correct.

From page 194,

Passenger compartment fuse panel / power distribution box

The fuse panel is located below and to the left of the steering wheel by the brake pedal. Remove the panel cover to access the fuses. To remove the fuse panel cover, turn the panel fasteners counterclockwise.