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I believe the horns on these are a low-side switch (i.e. switch ground, not power) through a relay ... have you checked the relay, the button, and ground continuity at each location?
That looks like an aftermarket horn which requires a separate ground and power wire. The steering wheel horn button provides a ground to the horn relay which then powers the horn directly from the battery.
Haven't ya ever noticed that only one wire connects to the stock Ford horn??? That should have been a clue.
HONK!
.. if your horn isn't working, the bridge over/thru the steering coupler ("rag joint") could be broken. In that case, install a jumper wire over the coupler. EZ.
Does it have power? Does it have a ground? Then this is your problem. Power does nothing without a ground, and a good ground does nothing without power. YOU NEED BOTH!! Make sure that the horn is grounded, powered, and switched. You do not need to build rockets in your spare time to work on these but since the technician today goes for his MATCO scan tool before anything, he has lost trackof the fundamentals of basic operation. Good luck.