Try adjusting them. The adjuster moves the contact......a horn is nothing more that a buzzer.........which essentially is a relay coil in series with the contacts. You want BIG power to them, NO12 wire and good solid connections. Don't rely on alligator clips. Clamp in a vise, bolt up the ground, get a tool to fit the adjuster, and when you are ready, hook other lead to a good battery. Wiggle/ twist the adjuster back and forth until you hear something, turn it in and out at least 1 turn either way, and if it starts to work, adjust for best sound / pitch.
There are people that have taken them apart, not me, LOL