Well I started to give you some suggestions but stopped in my tracks..............how exactly did you "power the horn"? If the horn is still wired per original, it has two wires running to it. One is live voltage from the ammeter and the other is ground back to the horn button which does the grounding. No relay. So did you just jump the horn ground terminal right to the horn bracket and left the power wire attached? Doesn't seem like the dedicated ground wire touching ground or horn parts touching ground will affect the ammeter. Hard to "short out" that wire and cause ammeter trouble. It wants to short out...... to honk. The fact it honked and went dead is not a hot wire short. The horn ground wire has a connector somewhere. But many things get changed over the years.
Jerry is suggesting dirty positive or negative terminals a some draw tests on the ammeter. (headlights). Observe what the ammeter does. Do that stuff.
If you reply about how you did the test, I can give a comment about what is wrong, and why ammeter "doesn't move". But as you can guess, I need to know what that means.
If the hot wire to horn shorted or the horn guts shorted, that could damage the ammeter. You may want to disconnect the horn hot wire at the horn while doing things, and of course see if that wire is hot at the horn.
I hope you are not forgetting to tell us the reason the battery was being re connected is that you were working on the horn or some wiring
