That reminds me - there are 2 different side/headlight stalks - one with an earth tag (1/4 inch push-on) and one without. IIRC, this tag is only for the windscreen washer pump. I don't know why, but on the one with the tag, the washer pump is fed 12V by the stalk (black wire I think) and then instead of running a wire from the washer pump straight to earth, the wire returns to the stalk (black/blue) and then to earth via the tag (brown wire)(brown is always an earth on all circuits). It's easy enough to rewire the washer pump so it doesn't return back to the stalk.

With regards the horn, as standard 12V is fed through the coil side of a relay and then to the column, so the horn push brown needs to be earthed.