Do you mind my asking how you knew there was actually something wrong with that part of the loom in the first place?
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Hhmmm. If I understand you correctly… doesn't seem to be a return to the alarm on mine? Just a single wire/connector to the boot switch…
To be able to get to the sensor you need to remove t-bar. Problem is, in order to remove t-bar, you need to remove both seats.
Until you get it sorted, just unplug the horn connector under the bonnet instead of having to disconnect the battery every time.
Try this- https://www.the-t-bar.com/forum/21-david-aiketgates-guides/11201-alarm-system-how-to-test
Have you checked boot lock once again?
Second thing, horn should not be the one that is triggered by your alarm, but BBUS unit (Batter Backed-Up Sounder). If you disconnect it you should silence the neighbour wakening device.
I already answerd on .org ...
How did you run the diagnostics?