I'm trying to wire another switch into a already existing circuit. My understanding is that it would have to be a fourway switch. One threeway switch is at the top of my basement stairs and the other is at an outside basement door and the new switch would be between these. Power source is at the basement door, the switch at the top of the stairs has all three wires connected to the switch. Would it be better to break in to the line that goes to the switch at the top of the stairs? Can anyone email me a diagram ?