IIRC, mine just pulls off. It's held on by several spring fingers. Just try to grip it evenly all around and pull. "78 might have been different but I doubt it.
Two possibilities for it not working. Most likely is the horn(s) themselves. There's an adjustment screw on the back which changes the gap inside. With 12 V applied to the horn, try running the screw in and out to see if you can get any noise out of it. The horn is basically a vibrating diaphragm and the gap is for tuning it. If the horn doesn't work and the horn button is applying 12 V to the horn and it still doesn't work, then it's the horn.
If that doesn't work, the next place to check is the horn relay. Not sure where it is. While someone is pressing the horn button, check for voltage at both the horn and the input and output of the relay.
Lastly is the contacts in the steering wheel itself. Check those for oxidation and polish w/ emery cloth if they look crudy.
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1973 L-82 4 spd