[Jack21]

I'm a reasonably decent automotive electrician so I'll give this a try. Petek is another good electrician.

Relays are used in several locations on your car, and the horn is one. A relay takes a small control voltage to activate a relay. The power side of the relay takes battery power and operates a device.

It helps to draw a picture of this, then refer to it during assembly.

The control side of the horn is called "complete the ground". The control side of the relay is always hot (term 85). From the BAT lead on the fusebox. Term 86 completes the ground through the horn button. The horn grounds through the steering column, which is grounded through the frame. You can reverse these two leads and it will still work.

Your black 18? Does it (a) have +12v, or does it (b) show continuity with the frame when the horn button is depressed? By your description, it goes to the horn button. So term 86 goes to black 18. And term 85 must be connected to an always hot power source.

On the power side, term 30 goes to a good power lead, and term 87 (black 21) powers the horns. The power side is the same source used for high amperage devices like fan, lights, MSD, etc.

The relay has no internal, or case grounds, but the horn does. If you bolt the horn to fiberglass, you need a ground wire from the horn case to the frame.