Does that mean the fan isn't running at all or it's running and the air is blowing out somewhere else? If I'm to assume the fan is running and the air is only blowing from the defroster onto the windshield, couple that with the rough running and I'd suspect a vacuum leak in a hose under the hood. A vacuum leak can cause rough running, typically along with a high idle speed. With a loss of vacuum, if your heater controls are run on engine vacuum, that system will be spring-loaded to go to the defrost setting for safety. Your feet might freeze but the windshield will be clear.