me and the hen was watching some youtube and the truck horn just went to sounding off. it kept blowing soild, she beat me outside and it stopped when she opened the door.
started blowing again and kept going till i took the ground wires off both batterys.
looks like it will be fun even finding the horn, must be under the battery box on the pass, side.

A little internet digging, seems to be a common issue. Sometimes its water in the fuse block/relay center in the cab. Most often its the air bag module on the steering column.
Here's a good discussion, albeit kinda long.

Ford horn blowing constant