Is the fan working or not? Probably either the fan blower or the blower regulator if not. There's a three prong plug on the firewall right above the brake booster that supplies power to the blower. If you unplug it, you can check the voltages being sent to the regulator (which in turn controls the blower).

If the fan is working, then you might have a vacuum problem, but the thing with that is that certain flaps spring open when the elements or vacuum supply fail.