Problem solved
There is a small "L" shaped headliner that wraps around the front of the head which is held in place with velcro. After removing it, I fished around behind the face of the microwave cabinet with a piece of coat hanger and snagged the wires for the horn. I tested the wires at this point and had power yet 3 feet away at the horn itself... no power. The problem in the end appears to be that when the deck was assembled to the hull at the factory, the wire between where I was testing and the horn was inadvertently sandwiched between the top of the head ceiling and the deck of the boat. After one season of hull/deck "flexing", the wires wore through and shorted out.
I replace that section of wire and everything works fine.
I love a challenge.
