The PO would have to have installed an internally regulated alternator if there is no regulator on the firewall, so it's probably OK. A turbo-hydro could look, and be, 40 years old.
The horn wire supplies ground to the horn or horn relay when the button is pressed. That wire goes to the horn and the othe terminal on the horn needs to be hooked to a +12V source. Both terminals on the horn must be isolated from ground, except when the horn button is depressed.
