99 Toyota Avalon’s horn works intermittently, seems to like the winter better than the summer. In the Summer it barely works and works more so in colder months but it is still not dependable. Replaced the relay fuse but that did not fix the problem. normally I don’t use the horn so I thought it was because of that reason. thoughts?
More than likely the problem is with the membrane switch in the steering wheel for the horn.
The membrane switch is part of airbag for the steering wheel.
So when the membrane switch starts acting up, the air bag is replaced.
Tester
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Horns are located in places where the sound can get out readily… and rain can get in just as readily. They “ground” through their mounting, often bolted directly to the radiator support.
That electrical “ground” is prone to rusting, and many do. That creates an often intermittently high-resistance connection. Since resistance “drops” voltage, the 12 VDC horn has to operate on what’s left after the connection eats some of the voltage up. That rusty connection can also vary based on the weather.
Remove the horn, clean the “ground” surfaces as well as you can (try to get the rust off) and remount it with new bolts. If that doesn’t work, the horn itself is probably rusted internally. It is, after all, a metal diaphragm connected to a solenoid with an iron core. The good news is that you can replace it with a generic aftermarket horn from any parts store. It’s been years since I’ve bought one, but they used to be about $12. They’re probably still less than $25. Or maybe $35. The point is, a replacement is cheap.
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A shop can use test leads to temporarily power the horn directly from the battery. If it beeps then, the horn itself is working properly, and the problem is somewhere upstream. Problems upstream usually involve either the horn button on the steering wheel as posted above, the clock-spring (the flexible wire component in the steering column allowing a rotating steering wheel to be electrically connected to the rest of the car) or the horn relay.
I wonder if any new cars come designed with an rf link between the steering wheel and the rest of the car, in order to eliminate or at least reduce the complexity of the clock-spring mechanism?
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Interesting question, George. I’d bet that the horn will eventually be controlled through the Body Control Module like the windows and so many other things currently are. It probably already is on high-end cars. 
@the_same_mountainbik The horn is already controlled by the BCM on cheaper cars. My '07 Jeep Compass is like that. The horn is linked to the remote lock, as well as the panic button. Ironically that setup made it easier for me to trace a fault when the horn stopped working. Since the horn didn’t work either from the steering wheel or from the remote lock, I could rule out the clock spring cable right away.
Macfisto, I guess I need to get out more… 
Thanks for the tip.