Try this;
1. Get in the truck and fasten your seat belt
2. Put the key in and turn to on, don't start
3. Push the lock button on the key fob for at least 4 seconds but not longer than 10 seconds
4. While holding the lock button down, press and hold the unlock button on the key fob
5. When a single chime is heard release both buttons and the horn should be deactivated
Do the procedure again to reactivate.