Heater/AC Control Problem

My '71 Valiant's Heater/AC control isn't working when I hit the defroster button. Everything else works great. Feels like the button is "sticking". Is there a fix, or am I in the market for a new one? I could give a darn about AC or heat, but I need to defrost the windshield in the morning. My ears are freezing from driving with my head out the window.