The horn is not fused. The only protection that circuit has is the fusible link at the battery. The 12 gauge red wire goes from the battery fusible link all the way to the horn relay by the gas pedal, and from the relay it goes back out to the horn on the 12 gauge dark green wire.
Here is the full wiring diagram.
http://67-72chevytrucks.com/vboard/s...&postcount=302