I agree with Geetee on the door parts and cables. Also you didn't say if it is an A/C car or not. A/C cars have a heater shut-off valve that blocks water from the heater core when temp is set to cool. Non-A/C cars don't have this valve (I think), so hot water is always flowing through the heater core. So, if the doors aren't shutting, then air from the cowl could be forced through the heater system. I have seen everything from leaves, sticks, trash and even a dead mouse stuck in the heater plenums, holding them open.
PAWN SHOP FIND
RESTORATION IN 1980...
"You can have it all, my emipre of dirt. I will let you down, I will make you hurt"