I can not explain why they was no continuity to ground on the splice into the yellow wire at the horn but it still managed to provide a ground for the compression.

Splicing into the blue/black wire at the horn seems to have solved the problem.

Edit: splicing into the blue/black wire only solves the problem when I leave the original horns disconnected. Once they are reconnected the compressor activates.

I would rather be doing something simple like changing a head gasket or a rear main seal (grin)

Randy