For mine, after hours of online and manual research and probing around, it was a very simple fix. I found the 6J4 information in the manuals as referenced in the images above, and I found the correct wire which was tucked in the dash under the radio. I lay down on my back under the dash, found the coils of wires, and I identified the jumper cable for 6J4. It's a really thin black wire. Whoever removed the siren switch simply clipped the jumper and taped the cut ends with electrical tape so that they didn't actually connect. However, for the horn to work, the wires have to connect. I simply unplugged and removed that section of jumper, and then all I had to do was to push the male and female spade connectors together without the jumper section. The horn instantly worked. Hope this is your issue.
See also
tahoeyukonforum
.com/threads/horn-problems.80217/#post-994841
(Too new to post a real link apparently, and forum software too ignorant to realize it's an internal link)