FWIW You might try....
running a temporary dedicated ground from the battery negative terminal to the horn. Run it that way for a while and see if that fixes the issue.
If that doesn't work try the dedicated temporary ground attached to the steering column.
I had a similar issues with my 65. I restored the column (bearings, bonnets, and turn signal switch) and used a different steering wheel and the problem went away.
_____
John
