I had no horn when I first bought my '77 as well. After jumping the horns to the battery to ensure they worked, checking continuity of all the wiring and the relay, and determining that I had power at the wheel, it was this forum that led me to check the ground at the steering column rag joint. I used a short jumper wire to test it and my horn worked great. So I made a permanent ground jumper. Takes 30 seconds to check and about 10 minutes to fix permanently. If this is your issue it's a simple check and fix. Good luck!

 

My '77 was missing its relay. I couldn't find one at the junkyard, and I'm too cheap for a new one, so I grabbed a modern three terminal relay and socket, crimped male spades on the wires and plugged it on in, and had a working horn.
Within a few weeks it stopped working again, and I traced the problem to the jumper wire at the rag joint.