New horn installed, now electrical problems

[Bcontheroad]

I have a 31 that runs great.
I installed a new horn. Now the car won't start.

Key off, 6 volts to both sides of the starter fuse. Turn the key on, voltage there gone; won't start. What did I fry? Fuse is fine.

When key is turned off, voltage reappears but horn won't sound. Disconnected the horn. With key off and horn button pushed 6 volts get to the horn wires. With key in on position, no voltage to horn wires.

Any description of the method to solve this is appreciated. I am good with reading wiring diagrams and using voltmeters. But I was hoping it was something simple like a new ignition switch...