Exactly Alfred explanation is the most accurate.
Horns circuit is hot every time ignition is on the RUN/ON position. So the cable black/yellow gets energized then. The circuit is completed each time you press the horn button giving the GROUND needed to make them sound.
As suggested by previous members disconnect the horns cables going to the horns itself. In some models is just a spade female connector, on others is a “O” shaped connector. Tape the cable black/yellow which is powered. And start your circuit testing by confirming that there is a ground signal on the cable black/yellow/pink by doing a continuity test to any chassis part of your car.
As asked by Alfred your horn ring behind the steering wheel may be defective or been in the wrong position since is closing to ground all the time. I would remove the steering wheel and disconnect the horn oring switch from ground and see if the horns still receive ground at their terminals. If ground signal is no longer at horns once removed the steering horn ring switch, then your problem is there. If horns still have ground after this, the problem is Elsewhere on the wiring harness.
Study carefully the switch outside the car, see if it’s well assembled and test its switching function with a multimeter.
Have fun with your electrical criminal investigations and find the culprit for the self activating horns....
Best regards
Lpeterssen
