First, put 12v directly to the horn- it should work loudly. If not, your horns not working. A common problem on old horns is that they get rusted/gumed up inside. Often this can be fixed. Most horns have an adjustment star on one side of the diaghram. Find this "star" or hex screw. You may need small vise grips, but turn it 1/2 turn left and 1/2 turn right to restore the contacts inside. The factory uses this to adjust the maximum loudness, you can too later. The moving of this adjustment, even though you have brought it back to "center" is enough to restore the contacts inside. Try the horn again, it should work.
