My horn's not working (66 Chevelle). I have read quite a few posts on this subject and am working through the problem. I grounded the black wire terminal on the horn relay and nothing happened. The black wire terminal shows to be hot when I test it with my lite tester. Is that correct? Should it have power at the ground terminal? I applied 12V directly to my horns and they work fine. Trying to figure out what to do next. Hoping it is just a bad relay.
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If the horns work it's most likely the relay. Itentify the two relay coil connections. One will be +12V and the other goes to ground through the steering wheel (I would guess you knew that). If you ground the negative side of the relay coil the horn should blow or you should hear the relay click. If the horn blows replace the horn switch. It should be an easy fix.

Good luck
Chuck

Like Chuck said. If you are not sure ,you can take the cover off and follow the coil of thin wire back to where they attach to the terminals. Gary