I have a Ruud and a Rheem system that I am adding new thermostats to. The Ruud was very easy to add the C wire to. I just found the blue wire on each end, connected it to the C terminal and I was done. On the Rheem, I am not sure as to which terminal I am supposed to use. When looking at the diagram, it goes as follows: C - C - X - R - Y - G - W. I thought originally that the X wire was the answer, but when I tapped into that wire, it did not work. Also, the diagram shows that the brown wire is supposed to be plugged into the second C terminal, but when I opened it, the brown was plugged into the first C (the first one on the right). Could anyone give me direction as to how to * Any help would be greatly appreciated!!