The horn circuit has a button that supplies ground, a relay that uses the button to switch the horns on and off, and two horns. The relay reduces the current that the horn button has to carry. This makes it possible to use a small and simple switch and slip ring on the steering wheel.
Note that the horn ground is built into the horn mount bracket, so the horn must be mounted on metal with a good connection to the car body.
The horns typically develop an internal short to ground, which causes the fuses to blow.
The horns are located under the car on the driver's side, forward of the front wheel well. Disconnect BOTH horns, replace the fuse: if the fuse blows immediately, the problem is most likely in the cigar lighter circuit. If not, then press the horn button on the steering wheel: if the fuse blows the problem is in the wiring between the horn relay and the horn. If doesn't blow when you press the horn button, connect ONE of the horns and try again. If it still doesn't blow the fuse, connect the other horn. Chances are that the first horn you re-connect will blow the fuse. Whatever horn blows the fuse at this point has an internal short and is trash. Time for a trip to the junkyard...
Horn does not honk, but does not blow fuses:
Run a jumper wire from the battery positive terminal to the horn input connector: the horn should honk. If it does, then your problems are in the other parts of the circuit. If it does not honk, replace the horn.
Use a test light or voltmeter and connect one lead to the horn wire under the fender and the other to ground. Push the horn button: you should see 12 volts or the test light will turn on. If it does not, check the fuse. If the fuse is good, check the relay.
The relay will click and honk the horn if you short the Yellow/Lt Green wire to ground. If you have a test light connected in place of the horn it will light. If it does not activate the horn or test light, look for 12 volts at the Lt blue/white wire. No 12 volts at the Lt blue/white wire, you have fuse or wiring problems in the circuit that supplies the 12 volts to the relay. Good 12 volts at the Lt blue/white wire and the Yellow/Lt Green wire shorted to ground and no honk or flash of the test light, replace the relay.
The rubber foam usually deteriorates over time and that is what makes the horn switch fail. Take it apart and replace the foam with some from packaging material. A little creative engineering may be required, but you can do it.