Under the tank, on the left hand side, is the hazard plug, with three wires, brown, white/blue and yellow/ violet. On the right, inside the big plug, is the turn signal cancel switch plug, brown/white. The horn switch is not separate, the lazy way would be remove the horn relay, probably to no effect.
The retaining tabs are old and brittle, so go easy.
If pressing the horn at the same time as the indicators, watchya got? The turn signal cancel at the same time as the indicators, watchya got? Long shots but quick.
And there are some other suggestions you havn't reported back on yet.