My horns didn't work when I got my truck. I took a razor blade and slipped it between those vibrating (bakelite?) plates. That brought the horns back to life. They must have been stuck together.

This may not be a solution since yours were working. You have two side-by-side, right?

1948 3/4-Ton 5-Window Flatbed Chevrolet

34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.