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This coopery guy has the remnant of a bullet connector in the tunnel-like socket.... horn related I think. what should plug in, an earth or a positive of some sort. makes a nice image.
That's the grounding tab for the horn. The wire should be purple/black....but I'm only two sips into my first cuppa coffee so don't take that to the bank. If you find the wire and plug it in and the horns blows...well...you'll then know why it was unplugged
From personal experience, I can tell you that you should remove that contact (let's call it a brush), clean it and clean the rotating copper part (let's call that an armature). Then reassemble using di-electric grease. If not, that contact against the armature makes the most grating, annoying, grinding sound whenever turning the steering wheel. Which is about all the time.