Hi LC, First check the 'Horn' fuse is not blown or poorly connecting.
If not the fuse, disconnect the wire from the terminal (I am assuming it is one terminal) and with a piece of wire from that horn terminal, touch the other end to the battery + terminal and see if it works. If it's a two terminal one then you will need to hook up two wires to it with the other one to the body of the car or the battery - .
If it doesn't work, there is usually a small screw partially screwed into the back of the horn. Note its position and gently rotate the screw back and forth and put it back to the original position. Try with the wire again and it SHOULD work. If you can fix the wire safely to the horn and battery terminal (perhaps get someone to hold it for you) while rotating the screw, this can work better as you then start to get sound from the horn.
If the horn works when you try the wire first off, then the fault could be with the switch in the steering wheel. Often they get corroded through lack of use. I have found that repeatedly pressing the horn by drumming on it (if you see what I mean) can help start getting some sound. Try this with the horn plugged back in, but check/clean any corrosion around the contact to the horn.
If neither of the above work, then it's a job for a multimeter and go through methodically checking each end of the wiring. Not sure on that truck if there is a 'horn relay' if there is, make sure it is plugged in properly, or try swapping it for its neighbour.
Good luck, I have rarely needed to do more than step one.
Richard