What is that passing on the basic S30 wiring diagram and doesn't it show the horn wires going to the steering wheel ?

JDM cars had flash to pass (possibly in Europe, too?). The horn button grounds the horn.

The issue may be that you're not looking at a FSM wiring diagram. It is shown clearly on the 72 & 73 wiring diagrams. Look for a wire labeled GB.

That is doesn't look complete. The diagrams I have show 4 wires going to the passing relay. A red wire, red with white stripe, black, black with yellow stripe.

ColorWiring240Z-1971Early.pdf