The horn is probably the most simple circuit in the truck. The 15A Haz-Horn fuse supplies constant power to both the coil side and the switched side of the horn relay. The other side of the coil in the horn relay runs to the horn switch (steering wheel centre pad/airbag). When you push the horn switch, it closes, completing the circuit by grounding the coil side. This closes the relay, supplying current to the horns, which are grounded to the body.
Troubleshooting
1: Do your turn signals/hazard lights work? If so, the fuse is good.
2: Do you hear the horn relay click when someone presses the horn switch? If so, the problem is between the switched side of the relay and the horns (see 3a). If not, the problem is between the fuse, the coil side of the relay, and the horn switch (see 3b).
3a: Disconnect the wire from the horns. Do you get 12V when the horn switch is pressed? If so, the horns are bad (rare) or they have a bad ground. Unbolt the horns, test them directly on the battery. If they work, clean up the brackets/bolts and ensure a good ground. If you don't get 12V, it's time to start tracing the wire back from the horns and looking for a break (rare).
3b: Pull the relay. Do you have 12V at two of the terminals (power supply for the coil and switched sides), ground at another when the horn switch is pressed (triggers the relay coil), and resistance on the fourth (power wire to the horns)? If one of these is bad, it's time to start tracing wires and looking for the break.
Hope that helps!