Here's what solved my intermittnet horn issue on my 2000 Toyota Sienna:
I removed the air bag, the steering wheel, the clock spring and the multi-switch (it operates the cruise control and the windshield wipers). It was quite a process. Underneath all of that is a ring of ball bearings - they were badly rusted and dirty. I sprayed a solvent on them until they were shining clean. Carb cleaner will do. Then I sprayed on WD-40 to lubricate and to inhibit future rust.
The horn works now.
That ring of ball bearings is essential to maintaining proper ground, which is essential to the working of electrical components - but you must remove many things to get to them.
That was the solution to MY problem. Other problems could be fuses, relays, wiring, or (rarely) the clock spring.
Good luck.