73 duster horn relay ?

It appears to me that (tiny red) the coil wire going to the horn button wire has burned off. Look around the coil for a "piece" of a tiny red sticking out. You may have to replace the relay. What might have happened, is that the horn button was shorted and held the relay engaged. Someone might have unplugged the horns to stop them from sounding. This would have created a drain on the battery, but eventually would have over-heated the relay and burned off that wire.