So I assume you bought a car ignition switch since it has three prongs. This means that one turn of the key will be momentary (when you start an engine, you hold the key in a position then it springs back to on). So, get a multi meter or just test the connection so you can find the two contacts that are connected when the key switch is in the permanent ON position (NOT the momentary position). Then, cut the red wire in half wherever you want to mount the switch (positive or which ever goes to positive battery terminal), strip both ends, and screw them onto the points two contacts you found.