2013 Ford Taurus Rear Defrost Issue

Hello, I recently purchased a brand new 2013 Ford Taurus SEL AWD.I live in Colorado where it can get downright bitter cold which is what I am dealing with now (0 degrees out). I noticed that my rear defrost works but it only works right in the center of the rear glass. It seems the sides don't melt the snow like it does for the middle. It's nice that it creates a little window to see in the middle but of all Fords that I have had, it defrosts the whole glass and quick. My car is a little more loaded so it does defrost the front mirrors. The car is under warranty. But just wondering if that's normal now for Ford? My previous vehicles have been 02's and 06's.