Current drop
 

If you can blow one horn and not two I would say you have corrosion on a
connection limiting the current flow to the dash. Watch the positive
voltage at the dash while you blow the horn. I bet it is dropping to 6 or 7
volts. You can do this same voltage test with a load at different locations
working toward the battery. You might just find a caroded inline fuse
holder or something like that. Good luck Neal