On my car the wiring for the second horn was taped up next to the wire leading to the single horn. I had difficulty getting enough free to reach the second horn and had to make a short extension piece. The lack of free length may have been because the wires had to fit around the aircon radiator.
Be careful to set the horns so that any water that enters can drain out. They are mounted in a rather vulnerable place if you enter standing water. I managed to drown a set of new FIAM horns that never really recovered. As luck would have it I had found a horn of the same type and in the tone to compliment the single horn originally fitted by Morgan so I have recently fitted those.
I can't say if the GDI has the same wiring but the mounting hole for the second horn is probably already in the panel. It is a bit awkward to get fingers in above the panel to fasten the the horn but it can be done.
Last edited by Gambalunga; 28/02/16 10:10 AM.