This morning I tried the time-honoured method of hitting it a few times with a rubber mallet, and Hey Presto! It's working now! Diaphragm rusty, perhaps. So now, for the first time in perhaps forty or fifty years, everything on the car works: horns, radio, clock, heater/blower, all the lights - it's all good. I went for a celebratory drive just now and honked at people!