So I need to run a switch that cuts one line and powers another from the battery.

If it helps I'm trying to override a fan control unit with adjustable temperature.

I'm trying to make it power on the relay in from the battery and the line I need to cut is the temperature sending unit that has 2 wires going to it one is +12 and the other will "ground" out as the temperature moves up. The ground out side is the side I need to sever the connection to (so it thinks the engine is HOT)

would'nt a 3 position toggle work for you? Pos. 1 (thermo), Pos. 2 (off), Pos. 3 (on) Or even a 2 position where you can run it on the thermo control on position one or full on with position two. The 3 way would be nice if you ever do any water crossings. Add another relay if needed.