Because I'm incompetent at electrics (at least as they exist in DeTomaso cars), the only way I solve such problems is with a VOM, starting at one end. When electrons stop showing up on the meter, I usually find the problem. When our horn stopped working 20 yrs ago, I did this because to qualify for Concours-type car shows, everything must be working.
I found the horn stalk button gets power thru the turn signal switch wiring, runs down through the stalk, turn signal switch and steering column, thru the front body wiring loom, goes to the horn on the front left below the headlight, comes ALL THE WAY BACK thru the body to the turn signal switch, up the switch stalk and is solder-grounded about halfway up via a raw hole drilled in the stalk. Facing away from the driver, there's a tiny painted-over lump of solder.
My horn ground wire was bowstrung-tight so sharp turns of the wheel pulled on the connection, eventually cracking it & opening the circuit. Yours could be the same but may be making contact inside the stalk- or somewhere else. Why DeTomaso didn't just ground the horn 2" inches away and save about 8 feet of wire & an hour of wire routing, I don't understand. Italy.....