Lets start with the horn wire you mentioned in your original post.
Since the horn wire runs from the horn or horn contact on your steering wheel to a relay that is either a positive ground or a negative ground, usually a negative ground.
The wire you mentioned will run either to or from the horn relay or from the horn or from the horn contact on the steering wheel.
Since it sounds like you are rebuilding or restoring this car, you will probably need a new relay so I recommend you get one of those and some hook up wire.
14ga will probably work for the all the horn wiring you will need to do.
Once you get the relay take a good look at it and see if you can find its match under the dash, probably around the fuse block.
If you can't positively identify and old one, you may want to consider just wiring one yourself. This will include new wiring, about 25' should do it and Butt type Spade connectors with both Male and Female ends and a wire stripper/crimper.
If you haven't done anything like this yourself, it only looks complicated, and I'll walk you thru it so it's done right.
If you can't find an old relay, you will also need an inline fuse holder with a 30amp fuse.
Once you have identified what you've got now and have picked up what will need that I've identified here, write me back with the info I requested and we can go on from there, step-by-step, and
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