65 Dart Horn Problems

Yes, to question regarding will state of assembly help you isolate area of problem. No to wiring working that way. Horn is an open ground. Only "hot" wire is at the relay itself. Entire column wiring/switches are an open ground that must be completed. Only single wire I'm aware of is the one from the horn contact disk on the backside of the steering wheel and connected to the plastic horn switch you mentioned replacing early on and it would be a ground. If you've got a steering wheel puller I'd remove the steering wheel and set it aside. Plug in your relay and if horn honks I'd move on to my other test regarding unplugging the column connecter and testing per that post.