Cold Air Coming in through Glove Box

Can someone tell me what would cause there to be so much cold air flow into the cabin of my 2003 Ford Explorer from behind the glove box. it has been really cold outside here lately and even with the heat on it is still cold as hell inside the truck. This seems to be a common problem and I have seen several post about a recirc door being the culprit. How hard is this to correct and how can it be done?