The existing turn signal wires will reach back to the wiring harness when mounted on the new brackets. You have to drill a small hole in the fender on either side of the plate light assembly to pass the wires through.
I have the same set-up, and yes the instructions are hard to understand. I also have rather large hands so I had some trouble getting some of the hardware into some of the smaller places.
As for the turn signal wiring, I removed the reflecter from between the signals, enlarged the bolt hole slightly and passed both signal wires through that hole. But I'm guessing what ever works.