Assuming the horn has one connection to chassis and the other coming from the horn switch and the 12V feed via the fuse, a simple check would be disconnecting the horn and trying again. If the fuse does not blow without the horn, then there is most likely a short circuit inside the horn. If the fuse still blows without the horn the fault is upstream from the horn, making the problem slightly more difficult. If the switch is operated with the stalk there is many functions in the same area. I would guess that a stalk switch would be more prone to failure than the centre one.
Some cars have a relay between the horn and the relay.I would recommend having a look at a schematic, if possible. Will save you a lot of time.
