I have a 1978 Chevrolet C10 that it's horn doesn't work. A friend of mine that used a tool to test the horn and the horn does indeed work but it turns out that the problem might be with the horn contact in the steering wheel and somewhere along the electrical circuit of the horn. I replaced the horn contact with a new one and the horn still doesn't work. When I tried to put back on the steering wheel the horn contact made some sparks but the horn did not beep. What could my problem be?

 

Well being as its a 78' its probably got a wire fire in the making in the column (thus the sparks) due to dry rot, chaffing, and whatnot. Pretty much you are going to have to start tracing wires from the column to the horn and see if you can find the break. Might not be a bad idea to start looking at a whole new column wire set or look to start replacing all the wires with new wire in general.

 

I had a similar issue with my 86. It was simply a poor ground connection.
The metal housing of the horn is the ground . Remove the bolt that mounts the horn and clean the metal of the frame and the horn body itself to restore a good ground connection.