You're falling victim to the insane engineers. The horn relay is inside the Body Computer and is not serviceable. Since the early 2000's, the engineers don't know how to run a horn without involving two computers modules. This system is actually less complicated than those on some car models.
You can eliminate the horn switch wiring as a suspect by locating the Steering Column Control Module, (SCCM) and grounding terminal 1 in the 10-pin plug. That's shown with the red arrow pointing to it in the third drawing. If the horn turns on, there's break in that wire or the switch is defective.
The way the diagram shows the circuit representation, it looks like you can ground terminal 33 in connector C2280B at the Body Computer, and that will ground the horn relay's coil to make it turn on, however, all my training told me those two computers talk back and forth with digital signals. I'd be nervous about grounding a digital communications wire.
A better alternative is to use a scanner that can access both computers to see how far the "horn request" signal is getting and which modules are recognizing it. If you see the Body Computer sees the horn request signal but doesn't perform the task, for example, that would be the logical suspect.
Images (Click to enlarge)
Saturday, January 18th, 2020 AT 3:42 PM