Check out the original post in this thread: viewtopic.php?f=37&t=4000&start=380#p94153
This one was for a 2011 car that did not have a discrete ground lead. The newer cars are the same, but have a Power lead and a ground lead. Just figure out which is the power and which is the ground on both the wires from the car, and on the horns, then connect the correct wires to the correct horn connections. There is also a photo of what the pigtail looks like.
Power goes to power, and ground goes to ground.