It's been a while since I had mine apart, but I believe that the contacts are inside the horn ring. The contact on a spring that pokes out of the steering column always makes contact with the part of the horn ring it touches, and when the horn ring is tilted it makes momentary contact internally and grounds to the three metal contacts and back into the steering column.

You could try just shorting from the steel nut holding the steering wheel to that wire - the horns should beep. If they don't, the switch isn't the problem (or at the very least not the only one!). Some cars use a relay - my PV does - but it doesn't look original. Some are wired straight from the horns to that wire. Look at the other side of the steering box from the shaft to see where that wire goes.
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