There are slip rings and brushes for contacts for the horn/cruise. The brushes can be worn/cocked out of position. Have to pull steering wheel to check this. Also check the plugs at the bottom of the steering column for tightness. The steering wheel switches have been known to go bad also. Its possible to remove and clean these switches. The dealer sells replacement steering wheel switches for an arm and a leg.
It is also true that the third brake light on the cab has to be working, if it is burned out, the cruise dies since the cruise control senses the third brake light current to shut off the cruise control.