If you do manage to get to the horn, I think you should check for voltage there before you replace the horn. An easy test, and it might keep you from wasting time/money on the horn.
If you don't have a voltmeter or test lamp, you could try just touching a (thick) jumper wire from the battery to the horn's positive terminal. (Depending on how the horn connections are made, you might also have to provide a ground connection.) Be prepared to jump -- if the horn itself is OK, the beep will be loud.
And if the horn itself is OK, then you have to troubleshoot to find out where the break in its voltage path is.