This may take two people.
Hook up a test light to the horn wire, and have someone push on horn button.
The test light should only light when horn button is depressed.
If it doesn’t then find the horn relay (in fuse box) and you may need to get a service manual – to find out which wire is coming from horn button.
The most likely suspects:
Blown fuse
Horn (or Horns)
Horn relay (in fuse box, by steering wheel)
Horn button.
Wiring harness is also possible, but unlikely.
My experience has been bad horns only, to date – with exception I had a horn button on my ’77 Cordoba in early ‘80’s, which went off by self when temp got below 10’ F.
It sounded like a dog you forgot to bring back into the house, on a cold day.
If you need a horn, then pretty much everything Chrysler made from about ’66 to early ‘90’s will fit. Just keep in mind that some cars have LO and HI and some only have one (not sure which one).
If car only came with one horn, then getting the other with the short pigtail, is not hard either. Going with aftermarket or even adapting an off brand horn is not difficult either.
The hard only part I know of, is finding a new horn button/contact.