Driver's side gets a freeze plug, passenger side gets the nipple Shawn describes, and it is an interference fit like the freeze plug. There is a heater hose that goes from the nipple to the heater core to provide the source of heat, and it circulates all the time whether or not the heater control is "ON" or "OFF". The other side of the heater core, of course, goes back to the water pump on the small nipple just about the lower radiator hose connection.