You are not exactly giving us a lot to go on here. You evidently have a specific type/model of proximity sensor in mind. Why not say what it is?
Clear a reed switch is a magnetic proximity detector and only needs two wires.
Presumably what you did before was something like connect the output to ground, giving you two wires: the supply wire and the ground wire. Then you applied current from a current source, or a resistor, and detected the change in voltage at the power source end of the cable.
