According to available manuals and diagrams, 1946 "trucks" did not use a horn relay. Some cars with dual horns did. Truck relays came in after 1951. Chevy shows only info for cars and trucks. Truck horns had two wires, one to ammeter and one to horn button to be grounded. The dual horns have one wire to the relay. The horn is grounded at it's mounting. The button is no longer grounding a full voltage/amp load. It is operating a low voltage/amp coil inside the relay. This closes the relay contacts and that is where the full load is switched. I believe Chevy finally realized that switching the full load at the steering wheel was too "sparky" and a stress on the horn contacts at steering wheel.
The typical old can relay has three wiring screw terminals. Usually marked B H S. Battery Horn Switch. Then they had a wiring diagram printed on them using 1, 2, 3.
Those metal relays need a good ground at their bolted flange to body metal (chassis ground).

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