Doug, you are right, there are only 4 wires. From the RV side of the plug to furnace, there is One red, two whites, one grey. On the other side of the plug, the Furnace side when you look inside the furnace from the outside door, there is 4 wires: One red, two blue and one yellow.
I just want to make sure I can do the extra stat without compromising the safety aspect of the furnace. But from what I understand, I did not altered any of the function of the furnace, just made a disconnect like I would do with a switch.
