2003 Pontiac Aztek Heat Blowing

I've never had problems with my heat system but this year we've experienced the worst winter in 20 years. The heat/defrost only blows on #5 setting. 1-4 worked even last week but this most recent storm hit hard. It's turned up to at least #4 to thaw the car out with all the ice storms we've had but I don't drive with it on a high setting.