[CarWiz]

If I understand your switch correctly, the spring rubs against the brass rod to make contact. (Pinched between a coil.) I don't think any plating will stand up to the wear for very long. Especially gold. Chrome and Nickel may stand up but carry a high resistance for contact switches. The zinc on the spring will wear off pretty quick and brass rod will wear at the sliding points.

If the switch design is fixed, I would be more inclined to use a Beryllium Copper spring and skip the plating all together.