On different cars some of the things listed are true & others aren't. On the 1979 car there is a small temp switch in the air intake that supplies power to the pump. There have been 2 different temp ratings for this switch. Replacement pumps are inductive instead of direct shaft drive. Most newer pumps aren't repairable & water doesn't invade the motor, due to inductive drive. I am surmising that you have a weak heater, esp at idle!