The solid yellow wire that goes into the horn is not switched by the horn button at the top of the light switch rod. It is "hot" all the time.
The wire that has a tracer that connects to the horn goes up to the horn button, and the horn button provides the GROUND for the horn. Then it turns the motor and makes the sound.
Find a wiring diagram and study it. You are wise to check things out in advance. I use a ohmmeter without connecting the battery. You have too many "amps" available inside that battery to use it for testing (unless you have added a fuse).
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