Installed a new Nest thermostat yesterday. My setup is a 4 wire system. Green, Blue, White, and red. I have central air and gas heat. They had the Blue wire "C" into the Y for the AC. Since the Nest needs constant power, I had to use the "C" blue wire for constant power for the Nest. Now I have no wire for the central air. How difficult is it to install a "y" yellow wire so my AC will work?