I would appreciate help for a non-working horn.
I had installed air horns which worked well for 2 years then stopped working.
I then removed air horns and replaced them with a conventional Toyota horn but still not working.
I have checked the following and all are working well: horn fuse, horn circuit breaker (relay); new horn works with direct connection to battery.
I am thinking that it might be the wire from the steering wheel to the horn, or the horn switch under the the air bag.
Any suggestions?

I'd suggest checking the ground, and yes the wiring to/from the horn to the relay.