2010 6.4 L. Where are all ground locations? - F-350 Super Duty Ford Cars & Trucks

This vehicle should have 2 horns. One on each side. It would be unusual for both horns to be defective. Possible horn relay/horn pad or fuse problem. You can test the wiring with a test light for power and ground to determine if it is a horn or power/ground issue. Here is a generic picture of the horn locations.

Did you check voltage and ground at rear defog connector, purple wire should be voltage, black wire is ground. Check to see what your missing, defog turned on. If you have good voltage and ground, then suspect a break in the defog grid. If you can locate the break, the auto store should have some stuff you can paint over the break to allow continuity for the grid.
Also, you should be able to check grid continuity, ohms test. If no continuity, something wrong with grid.
Here is partial diagram.

behind the dome light

It may be a grounding problem in the wiring.. probably the turn signal switch that is located inside the steering column..These switches have plastic parts that can crack and close a circuit. You need a dealer repair as there are various sensors and sensitive wiring inside the column that controls deployment of the air bag.

O2 sensors do not have fuses as they are powered and grounded through the computer. if you have a code set for this sensor then you need to have the sensor checked or you can just replace it.

P0449 is a code for the EVAP vent solenoid. You can find the vent solenoid on a support at the rear of the fuel tank. You have a electrical problem if you have only this code in the memory of the powertrain module. May be it's a broken wire(most of the time) or a
short to ground wire.

check ground between body and batterie ground , body and engine ground.try with a temporary jumper to see if it will improve

Ground wire or connection probably requires renewal.

Purge is located under truck near gas tank. Look above driveshaft for round black plastic box with red and blue electric connector. Not the bigger rectangular one in front of tank. To test blow through, then add power, ground and you should not be able to blow through. Sometimes it gets stuck so be activating it a bunch of times sometimes fixes it. Your vent valve is located on upper intake behind throttle body with a red and blue connector

Check the voltage in pin 16 of DLC 3. Hook a bulb type 12 v tester between terminal 16 of the dlc and ground. The bulb should glow even if the ignition key is off. If the bulb does not glow check the fuse located under the glove box on the passenger side.