The spring is a resistor. There should be a center contact that slides on "spring". Typical would be one wire "+" connected to the center contact and the other connected to one end or other of the "spring" and to the center contact of the dash light. When turned probably counter clock wise the lights would dim and the spring would get hot but probably not red hot and turned all the way to the right would be like connecting the two wires to the dash lights directly. No ground should be connected to the switch/dimmer.
