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Not referring to a Miata specifically but most air direction is controlled by vacuum controllers. The defrost position is usually the default position in the absense of vacuum supply. There should be a small plastic vacuum line coming through the firewall to the engine for supply. Locate that hose and track it from it source and check it all the way back to the control switch looking for cracks, breaks or any other form of vacuum loss. Follow any lines teeing off as they may be going to a vacuum resevoir and can cause a vacuum loss if leaking.