how do the defrosters work in cars? I'm thinking that it might be related to my coolant leak. I have a small puddle everyonce and a while, my mechanic thinks it might be my water pump but I dont think so. I know the lights in the car are just idoit lights but when I have defrost on the light comes on and the expansion tank is dry, but if I dont put defrost on I have a lil bit of coolant in the tank. Any ideas??