Just a thought, but the horn looks to be an original 6 volt type. From my reading of your signature line posts, I believe your truck has been converted to 12 volt. Could be the voltage slagged some thing in the internals of the 6 volt horn or it just gave up.
You might read through the info (and somewhat comedic comments) of Bolter "bartamos" in his
reply to a similar on-topic thread.
I'm pretty sure there are other existing threads that discuss disassembly, cleaning and repair of these horns. If you can't locate one of those old threads yourself, let us know and someone can try and find one that may help you go down that road.
