I think I'm on to something. I put a voltage tester between positive battery and engine block and got 12 volts. So ground seems to be working. Nevertheless, I cleaned all ground points and tightened them.
I then put a continuity tester on the horn wires. Green and black. Even with the key off, I have continuity between the two wires. I then connected the tester between the chassis and the green hot wire to the horn. CONTINUITY again. Even with my limited knowledge of electronics I know this can't be normal. Something is grounding the positive horn wire to the chassis. I note that there is no point where the two horn wires touch ( the positive runs over to the drivers side and the ground runs to the passenger side.
Oops!!. Almost forgot, perhaps they may be fused together inside the horn connector or horn itself. I'll have to check that next. If that checks out I'll trace the horn positive wire back to the steering column and try to find out where it's shorting.
Jersey Jim.