I’ve managed to pull up a circuit diagram & although it’s a bit, no, it’s a lot of a squint, if the horn honks with the alarm, then the relay in the multifunction unit must be closed. This implies a circuit from the fuse, through the relay in the MFU, allowing the voltage enabled by the alarm to sound the horn.
The horn buttons are in parallel with the alarm & so the clock spring assembly should be checked.
You have a meter, set it to continuity & with one end to ground & the other to the header 0288 (K109), press the horn button & you should get a reading of the spring is working.
I can’t help you with the header whereabouts, I’m still on holiday & working on my phone!
M

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