I am going through a similar problem at the moment. My horn intermittently doesn’t work (and it’s always when someone cuts me off or does something worthy of a good beeping, but when I complain to someone about it and say “see, look” and press it, the bloody thing works!) I tried to see if different positions of the horn replicated the problem, but there didn’t seem to be any hard and fast rules about what caused it and what didn’t. The fuse is fine.
My clockspring was harvested from my husband’s VT for the purpose of passing a roadworthy 3.5 years ago (my car had the airbag light disease) and it’s just stayed there ever since, but given its age, it was a reasonable assumption to make that the horn problem is related to it. Through a bit of steering wheel manipulation we’ve come to the conclusion that it’s almost certainly the clockspring. I don’t know if you’ve ever taken one apart, but they have a fine ribbon cable inside them, much like the ones you find inside computers and printers, etc and there is no slack in them. The fine wires inside are prone to breaking eventually, due to the constant flexing when you steer. Each of the fine wires running through the ribbon controls something different, like the steering wheel stereo buttons, the airbag, the horn, etc. My horn wire is likely almost broken through.
Given you have the airbag light come on briefly when you start the car as well as the horn problem, I’m leaning very heavily towards you being right in the first place about the clockspring being the source of the problem. How is your cruise control? If it’s a bit dicky too then I would be even more suss on the clockspring. If you know the one in your VT is good, then switch it out and see how the horn (and airbag light) go. You are much better off doing it now, because if the airbag light comes on permanently then you’re going to have to find someone to reset that once you have done the clockspring or aside from the irritating trio of beeps every time you start the car, you will not have a functioning airbag.
Remember to disconnect the battery and give the car 15-20 mins to discharge any residual power so you don’t trip the airbag light anyway. That’s what I’d try given your symptoms and the fact you keep hitting walls with lout’s (entirely valid, educated and helpful) suggestions.
*Disclaimer: it was not until I had typed and saved that wall of text that I noticed that a) the problem did indeed end up being the clockspring and davs politely returned to update the thread, and b) this thread is 5 years old.
I apologise for pulling this post up from the dead, I’m not even sure how it happened, but stuffed if I’m going to delete my magnificently crafted and accurate response, because I won’t get that 10 minutes back. Hopefully someone, one day, will learn something from it and my effort won’t have been in vain! 
