This is a strange one, but may save some of you grief. Turns out there is nothing wrong with the A/C or the horn!! I installed Optima Red Tops last December, and when I tightened down the long clamping bolts, the one on the drivers' side turned down into the wiring harness. 8 months of driving finally got it rubbed thru and shorted out the wires. I cut off about an inch from the bottom of the bolt, repaired the two wires in the harness, and I've got cold air and a horn again!!!