I am not sure that is wired correctly. Generally there is the positive wire from the battery (for example) that goes to one side of the relay switch, and another wire that goes from the relay to the horn. The ground wire is the trigger for the relay. For example, the horn button on the steering wheel or just a push button would ground the wire, which completes the circuit for the relay switch and send power to the hors (which are grounded to the body).
Like this
https://images.search.yahoo.com/search/images;_ylt=AwrExlRkLrFhThsA5aqJzbkF;_ylu=c2VjA3NlYXJjaARzbGsDYnV0dG9u;_ylc=X1MDOTYwNjI4NTcEX3IDMgRhY3RuA2NsawRjc3JjcHZpZAMwcGJLWlRFd0xqS2g4dXVaWWJDOXpRUHZOVEF1TndBQUFBRFNKNEFCBGZyA3lmcC10BGZyMgNzYS1ncARncHJpZANwRGV4M2pmblMuNlRuX2I4XzJDVXVBBG5fc3VnZwMwBG9yaWdpbgNpbWFnZXMuc2VhcmNoLnlhaG9vLmNvbQRwb3MDMARwcXN0cgMEcHFzdHJsAwRxc3RybAMzNgRxdWVyeQMzJTIwcGluJTIwR00lMjBob3JuJTIwcmVsYXklMjB3aXJpbmcEdF9zdG1wAzE2MzkwMDEzODA-?p=3+pin+GM+horn+relay+wiring&fr=yfp-t&fr2=sb-top-images.search&ei=UTF-8&x=wrt#id=0&iurl=https%3A%2F%2Fi.stack.imgur.com%2FmWz9g.png&action=click
