How many terminals are on it? Is the case plastic or metal? George is correct, generally one set of terminals contains the 12V and ground which is made by pushing the button the steering wheel and grounding through the column, and then the other side's switched 12V goes through the coil and grounds at the horn body.
I have seen some relays that had a metal case and you can see the 4th terminal coming out from the relay and connecting to the case where it is grounded at the firewall or similar location. But this depends on year and application.
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