Horn not working

First thing I'd do is make absolutely sure (rust, etc) that it is grounded. then devise a good heavy alligator clip jumper wire, say, no 14 or bigger, so you can jump power to it. if that doesn't do anything, leave the power connected and twist the adjuster back and forth after carefully noting about where it was, rotationally. Often the contacts get corroded. Horns are quite simply a buzzer. Anyhow, if that works, you will have to adjust the pitch to the other one so they sound "right."