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Of the two wires coming to the horn(s), one should have 12V always when ignition is on, and the other is grounded when the horn button is pressed.
So, with your multimeter at DC voltage setting and ignition on, see if 12V comes to one of the wires. Actually, if the horn itself is ok, you will see 12V on both wires.
Then press the horn button. If the button is ok, one of the wires should drop to 0V.
If, with the button pressed, you have 12V on one side of the horn and 0V on the other, the horn itself is not working. I have managed to fix this situation by turning the little adjustment screw behind the horn. I guess the contact points within the horn corrode it the horn is not regularly used.