I managed to extend the wires in my mid-2012 MacBook. It is necessary to remove the left side I/O board to access the two screws securing the antenna wires on the left hinge. I then used scissors to cut the wire from the thin metal plate (bend away the two narrow metal legs from the wire and cut the legs but not the antenna). I guess one could also desolder the antenna wire from the metal plate in lieu of cutting. After doing this, the wires will have just enough length to reach the antenna connectors on the 802.11ac card.