The easiest way to check the circuit from the horn button to the relay is to use a multimeter. Pull the plug off the horn relay by the radiator. It will be the black(?) one. Put one meter lead in the plug. Put the other lead on a good ground....stick it in the battery negative clamp if you don't have alligator clips.
Now, put the meter on the fender where you can see it and set to read resistance. Push the horn button down and resistance should drop to zero. If not, you are missing a contact somewhere. It could be:
at the bulkhead connector,
the flat connector under the steering column,
the little metal base on the turn signal assembly that the horn contact goes into,
the small button spring that seats in the turn signal assembly,
the cam ring,
the plunger that sticks up and contacts the horn button,
or just a horn button assembly that is not grounded.
If you do not have ground at the black wire when you push the horn, then start probing from the bulkhead back. Make a little jumper wire to stick in the black wire's connector and connect it to ground. Start probing the connections I listed above and see if you get low resistance to ground.....both sides of the flat black connector, the little button underneath the cam that contacts the cam ring, assemble the cam on the column and test the plunger, etc.
The green wire carries twelve volts to the horn from the relay. If everything is good with your ground wire (horn button circuit), reconnect the black wire to the relay and have someone push the horn button while you check for twelve volts on the terminal that the green wire plugs into.
Also, if the horn has been replaced at some time. It may have two terminals. One has to be grounded. Old horns grounded through the body of the horn via the mounting bolt but a lot of newer ones do not.
How do I know this? Mine was missing the cam and the plunger, the spring in the turn signal assembly was collapsed so the button didn't contact the underside ring of the new cam, the horn relay was bad, and I had to get new horns.
The car never had a horn from the time I bought it in '93, so every time I have the battery hooked up to test some of the electrical I've been doing I just have to hit the horn a couple of times before I disconnect the battery.