Original horn stopped working.

I have confirmed fuse is still good. I have also measured 12V’s between the terminal and ground when the horn is pressed at the steering wheel, but when I installed a new after market horn that’s supposedly compatible to the terminal and ground no sound is made when I press the horn.

What would be the most likely causes of this type scenario? How can I detect 12V at the horn terminal when horn is pressed but not have the horn make any sound? What’s still “broken”?

 

You have +12VDC to ground, at the plug that plugs into the horn, when the horn button is pressed, yes?
What does the ground wire off the horn read to ground, in ohms? Is the horn bolted to good, clean, bare metal, with a good connection to the battery negative through the body metal? Is the horn's ground wire connected to good clean, bare metal? Is the ohms from the horn's negative all the way to the terminal that goes onto the battery negative read almost 0 ohms? Make sure the negative battery terminal is OFF the battery when ohming things out.
If there is no ground wire from the actual horn, which I've seen a number of times, the horn being bolted to good, clean, bare body metal, connected to the battery negative terminal is vital. That means it picks up it's return path directly through what it's bolted to.
Did you try connecting a wire directly from the battery negative and positive terminals to the new horn to see if it makes sound? Could the new horn be a positive ground version? The British used to have a passion for positive ground systems. Exactly backward from the rest of the world, generally. Why? Only they know.
What about the ohms between the steering wheel plug and the horn plug? Should be less than 1 ohm, preferably less than 0.5 ohms.

Just throwing thoughts out there...
Pat☺