wow I can't believe you haven't found out what was causing this yet but I'll guess I'll jump in and try to help you. My horn stopped working out of the blue one day and while troubleshooting I found that my connector on the horn wasn't seated very good. I don't know how it happened I just know my horn worked one day and the next day it didn't so all I had to do is just push the connector all the way back on. Now I guessing your problem is not that since you had it at the mechanics. But my question is do you know for sure if the actual horn works. Like have you tried to jump it to make sure it will still make a sound because you might actually have a bad horn. Horn's aren't that complicated you have a + and - wire when you press the horn button on the steering wheel it send either a + or - signal to the horn then it makes a sound try jumping it to verify it works if it doesn't make a sound then it doesn't work and thats your problem. so basically just put 12 volts to the red/yellow wire going to the horn