Weak/no horn fix
I can't say this will work for everyone, and I suppose it isn't really ZJ specific, but that's what I did it on, and that's where I'll post it. :-)
Anyway, I was getting only a very weak noise when I pressed on the horn, if any noise at all.
I figured on having to replace them after checking fuses and relays, so I removed them from behind the bumper and tested them with a battery and small jumper wires. One was completely dead, the other only gave a quiet crackling noise when powered.
I had planned on going to the pull a part yard for replacements, but as it has been raining a lot here, I wasn't too keen on lying on my back in the oily mud in the parts yard to get them, so I decided that maybe they were just gummed up inside, and sprayed the hell out of them with PB Blaster. There was also a little star-drive screw on top of the horns that I didn't know what it did, so I took it out and sprayed Blaster in the hole, too, and screwed them back in. I shook the hell out of them and poured the excess Blaster out, wiped them off, and tested them again with the battery.
Success! My horns are reinstalled and sound like new, with both low and high tones working!
Best part is, this fix didn't cost a dime!