Has your horn stopped or is it your steering wheel thats the problem? Ive had mine sorted under warranty loads of times, something to do with the contacts behind the wheel not touching when the horn is pressed.
only one way to test it really take the horn off and put a meter on the 2 pin plug and get a mate to test the horn if power there then it is the horn rather than anything else
The horn is working but seems to have lost its tone and sounds like its on its last legs, I suspet this happened after the service as the Fiat Agent washed the engine compartment so maybe water got into the horn which I presume is a Trumpet type