Heating my car

I assume you're referring to the engine temp and not the heat inside the car?

If so, here's the fundamentals:

While the car is running but sitting still, the water pump is running slowly and the only air flowing through the radiator to cool the coolant is supplied by the fans. When the car is driving down the road, the water pump is turning faster and air is being rammed through the radiator along with the help of the fans.

So if the engine temp drops slightly during driving, your cooling system is doing it's job. But. I've seen some people remove the thermostat from their engines and they will see the coolant temp drop too much. Which is enough to ruin gas mileage and prevent the heating system from getting hot in the winter.