Why does my rear windshield wiper fluid spray when I use my car heater?

Every time I turn on my heater (including defrost) in my 2003 Dodge Durango the rear windshield wiper fluids starts to spray and will not stop unless I turn off the heat completely...by the time I get to work it empties out my wiper fluid. I can't use the heater/defrost on freezing days because the fluid starts to freeze. How can these two things be connected?