As it only happens when the horn button is pressed ,check for a pinched or bared wire from the horn switch down to the horn itself .It must be shorting out to ground somewhere , failing that , disconnect the wire at the horn and see if the fuse still blows when you press the horn button.
Then you will be able to locate the problem .
Blessed are the cracked, for they let in the light. - Spike Milligan
"When you come to a fork in the road ,take it ! When you come to a spoon in the road ,take that also ."