Toyota Repair Problems? Ask a Mechanic for Answers ASAP
It is not likrely the horn that is bad. The more probable cause is in the horn conacts in the steering wheel or pad. It may be in the spiral cable also.
I forgot to mention there is a possibility that the horn relay could be sticking on. You may consider switching this relay within other like relay in the fuse box as a test.
Before you take it to a mechanic try switching the relays with another relay that is the same style and see if the horn stops blowing. You can wiggle the wheel and kind of bang on the horn pad/airbag to see if you could duplicate the horn sticking or try to make it stick by wiggling the horn pad. Other than that there's not a whole lot the average person can do to diagnose or test spiral cable.
Let me know how it goes. If you need a second opinion after taking it to a mechanic, if you cannot isolate the problem just post back to here and I will get back to you.