Greg,

Everyone is right on the money. You are right on the money too...there is a internal jumper you don't see that powers one side of relay coil.

The third terminal that has the small wire to the horn button provides a ground for the relay coil when you TOOT TOOT.

1-Power in (from batt).
2-Switched power out (to horn).
3-Ground for relay coil (thru horn btn).

This is an example, not a actual numbering sequence.

Stuart