[Bill Hanlon]

I don't have a schematic for a late 60's Pontiac, but most cars of that era did not have a fuse protecting the horns.

Try this:

Remove wires from both horns and make sure the wire ends are not touching ground.

Replace fuse.

Push horn ring.

Did fuse blow again? If yes, the problem isn't in the horns, but in the wiring to the horns.

If the fuse didn't blow reconnect one of the horns and try again.
If the horn blows the problem is in the other horn.
If the fuse blows unhook this horn, connect the other horn, replace the fuse and try again.
If the fuse blows again either both horns are bad (not likely) or the horn circuit is mis-wired.