Ekkkkkk, IMHO, I would not put cardboard in the front opening, this isn't a bread delivery truck. You would pin the needle in 5 minutes and have a very expensive engine rebuild on your hands.
1. Make sure you flap in the heater box is working correctly, most are not and rusted in a position. My flap is always in the flat or down postion, in my head Im always getting fresh air , even if its pushing through the core.
2. This isn't a 2015 Mercedes/ Honda, all modern cars work, TOO WELL. Your technology is 50 years old. Make sure you resistor is working as well on the high/low switch.
3. The core isn't plugged, or leaves and junk aren't blocking the flow.
If all else is okay, get her a blanket and hot coco.
I drive 5 ,000 miles a year in New York during the winter in my 70 etype. It works, but remember, your driving a motorcycle with a roof, by todays standards.
Good luck
GTJOEY1314
