I bought a pair (hi and low tone) 12 volt plastic horns off the shelf at O'Reillys 13 years ago. They sound like American horns from the late 50s through 80s.
The wiring scheme for your original horn was always on battery voltage to one terminal on the horn. A wire connected to the other terminal ran back up through the steering column and connected to the horn switch in the steering wheel. When the horn button was pushed it supplied a ground and the horn made noise.
More modern cars/trucks did it this way instead.
Always on battery voltage was connected to one terminal on a horn relay (usually mounted on the engine side of the firewall).
A second terminal on the relay connected to the horn(s) which had a ground supplied by the mounting bracket (some had a separate ground wire).
A wire connected to the third terminal on the relay ran up through the steering column to the horn switch in the steering wheel.
When the horn button was pushed it supplied a ground for the relay.
The relay pulled in and supplied voltage to the horn(s) making noise.
Here is the relay I used.
Horns like
this should work.
