Where is the horn relay for a 2007 Buick Lucerne?
You’ll want to take a look in the owners manual but I believe it would be one of those 1 inch cube like relays in the relay center under the hood. Things like horn, AC, headlights, etc. are usually there. Should be a plastic cover over it, maybe 4x6 inches. Most of them are all the same so you can look at the number on it and substitute another one with the same number as a test.
Aren’t there a bunch of relays on the firewall? You’ll have to look in the owner’s manual to tell you which one is which. Or Google maybe.
The horn relay is located in the under-hood fuse box.