Check the fuses, and the fusible links. They can cause this type of thing. The Light could be an indicator of a bigger problem, that might include the horn, and it could be the least of your problems.
Put a volt meter in place of your horn and have someone hit the horn button to see if you are getting any juice to the horn, if so the horn is bad (or the terminals are dirty). Start there and back track.
that the mfg. has hooked the air bag sensor up so it stop's the horn from working. This way when you try to go threw inspection, it will fail.
That will make you return to the dealer to get it fixed. It's prob. a safty feature.
Did you fix your horn?
Try finding the wires to the horn switch and shorting them together. If the switch is located in the same location as the airbag then I would suspect the wiring harness is bad.