A) I added a Stebel Nautilus inside the fairing of my 2012 XCT shortly after I got it. For pics, see http://www.billanddot.com/victory-xct/#11
in my gallery, and the next half-dozen or so pics. Keep in mind, for anyone doing this, that you want a relay, you want the trumpets pointing down, and you want to use at least 16-gauge wire for your work.
B) I cable-tied the relay to the hole that formerly held the stock horm, and this is how I wired it (using a typical relay):
30: to positive (red) battery terminal, with an in-line 20-amp fuse.
85: black OEM horn connector.
86: white OEM horn connector.
87: one of the wires (either one) from the Stebel.
Negative (black) battery terminal: the other wire from the Stebel.
C) As you note and some others have reported, I too get a quick flash of the turn signals or something like that during the rare instances that I use the horn. I believe this is the result of some voltage blow-back -- as part of a senior moment, I forget the proper technical electrical term for this -- seen by the fuss-budget ECM. I think this can be cured by using a diode somewhere in the circuit, but I also believe that this is nothing to concern yourself about. As I say, I've had the Stebel wired this way for more than two years now, and it sounds loud and clear every time, no problems, etc.