Air horns? My MC9 has electric horn by the steering box. I also have intermittent problems, but mine are caused by the buttons on the steering wheel. They don't have very good contacts in them, so you have to get them pushed "just right" to get them to work. The floor switch works well.
If you have similar setup, with electric horns, and not really air horns, then see if you can isolate it to the buttons or not. If you wiggle the button a certain way and make it work, then the problem is the contacts in the buttongs. If not, start by checking connections on the horns themselves. They will likely be corroded. Next, find the horn relay in front junction box. Check connections there. Short relay to make sure it works properly.
There isn't much in the system that can go wrong.
If you really do have air horns, then disregard this post, as it probably doesn't apply.