Not sure of location on a '79 model, but on earlier models, the horn relay is located under the hood on the driver side front wheel well. Should be a small unit with 3 or so wires to it. One should be a heavy red wire, which is the positive power wire to the relay. There may be other power leads off of the horn relay to other items under the hood, like the wipers. Before just changing the relay, check for power at the relay. Instead of a bad relay, you may have a bad fusible link. A fusible link is like a inline fuse of sorts, located outside of the fuse block. Suspect a bad fusible link whenever you have several things quit working at the same time.
