| Pull the connector off the horn itself and check for corrosion. If that doesn't solve the problem, begin by taking a multimeter and checking for voltage at the same connector. If corrosion was the problem, coat the connector with dielectric grease and reassemble.
Test the horn by connecting battery power to one terminal and grounding the other. The horn should sound--replace it if it doesn't.
If it's more involved, I would recommend taking it to the dealer. It will likely involve removing the steering wheel and disabling the SRS system. | 
Linear Mode
