Rick,

Have you looked very closely at the terminals, and even the bottom of the relay? Having no marks indicating what is what would be nuts.

My post will probably confuse you more. A standard horn relay would have three terminals, just like yours. The new horns you have are wired together to a terminal that is one side of a normally open contact inside the relay.

Another terminal would be power from the ammeter (or where ever) that would attach to a different terminal...this is the other side of the normally open contact that the red horn wires are hooked to.

The third terminal connects to the wire that comes down the column from the horn button on the steering wheel.

The coil inside the relay is already jumpered to pick power off the powered terminal. That third wire going up to the horn button grounds the relay coil, closes the contact and allows power from terminal two to pass through to terminal three which is attached by the red wires to the horns.

This is a guessing game. I would guess that the ammeter connects to the middle terminal and that the wire from horn button to relay attaches to the terminal farthest away from the tab with the red horn wires attached.

Three terminals....three wires. One already to the horns. One from power and one to the horn button.

The horn/relay assembly attaches to the truck somehow and that attachment is probably the ground itself.

Stuart