1970 Dodge Dart- No crank or start- Battery draw

I assume you meant there is "either an issue with the horn relay...or horn button."
I think pt70 suggestion is a good place to start.
Because when the power was hooked up, you could hear the relay clicking. Therefore the relay was getting flipped on (ie. power from the horn switch).
But the horn never sounded. So, that's the other part you'll to trace. Either the relay was not getting power from the main circuit, or the hrons weren't getting power from the relay.