It works slightly differently depending on whether you have speed control or not. I presume you do, b/c that’s the only configuration that uses a horn relay. First check fuse 16. The horn switch is part of the speed control switch ass’y, and grounds the horn relay coil (DB wire) when the horn switch is depressed. If your speed control functions normally, the ground connection is probably ok, but the switch could still be faulty. the horn relay should have a Y/LG wire attached. It should measure battery voltage when the horn switch is depressed, and that connects to a DB wires which go to the two horns, one on the left for the low pitch, one on the right for the high pitch.
So check that the horn relay is powering up the horns. If you got battery voltage to the horns and they don’t honk, either both horns are faulty, or their grounds are making a good contact.
The horn relay appears to be located inside the passenger compartment, under the left side of the dash, near the speed control gadget.