The fans only come when they need to to save on electrical power. There is one (or more) heat sensitive switches that will turn the fan one when the coolant temperature gets to a certain level. For the most part when driving, the car moving the air is sufficient. When the AC is used, it overrides the temperature switches and turns the fans on to assure there is sufficient flow through both the radiator and the AC condenser. If your heat gage doesn't move off cold, sounds like your thermostat it stuck open or the sender circuit for temperature indication is bad.