Let me add more detail.....
When I have the horn switch depressed towards the FRONT of the car, I have no problem.
When I have the horn switch depressed towards the BACK of the car, I blow a fuse when I honk the horn.
If what has been posted so far is accurate (i.e. the sound is created by using one horn or both), then this means that one horn is used when the button is towards the front, and the two horns are used when the button is towards the back.
From this I deduce that it must be a problem with the second horn in. If it were with the first one, then the fuse would blow in both positions.
Which horn is the second one in, 335Hz, or 400Hz??