What I call a protect plate is actually metal and not a phenolic disc that the grounded wiper rides on. The disc provides a smooth surface and keeps the wiper separated so it cannot touch against the dash light and power in terminals. It also has small notches a bump on the wiper falls into. That action forms sort of a detent feel for the knob at the courtesy, off and rheostat positions. The end of the grounded wiper passing into the notched out area of the plate and touching the wire terminal is what turns the courtesy lights on.