Pretty common issue on the Carina AT19X and even the AT 21X - especially if you park the car under the sun always. This is due to the degradation of the sponge that seals the flaps inside the air ducts.
Basically, when you change the direction of the air flow, flaps within the ducts open and close off passages to direct air out through the selected vent. These flaps have a spongy/foamy border so that it seals off the duct, preventing airflow. When the seal is degraded, it doesn't seal properly and cold air flows into other ducts and out through (in this case windscreen) vents.
You will need to take the glove box out and inspect the ducting and flaps to make sure they seal properly. If the sponge is okay, you might need to adjust the cabling so that the flap tightly seals the ducts when you change air the flow knob/switch.