Holes or gaps in the floor boards or firewall area are fairly common but can be tough to find. One trick is to put a bright light inside at night and then go outside and look from underneath to "see the light". Sometimes it works in reverse too - put the light outside and under the front end, then look from inside.
I've also seen a floor heater outlet that was not being closed properly when the heat was off, allowing outside, unheated, air to blow through.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL