This comment triggers something in my mind. Have you heard of "exhausters"? They are used to release the positive pressure from inside the passenger compartment while the blower is working. If you put a fan on the top of a jar and turned it on, it would fill the jar with air, then when the jar is completely full of air and can't hold anymore, there will be a back pressure against the fan as there is no more room to put more air into the passenger compartment. So to eliminate this effect in a car from putting similar back pressure against the blower when the interior fills with air, they put what is called "exhausters" somewhere on the car to release pressure from the inside of the car to the outside so the blower can keep pushing more air into the car. The exhausters can be in many places and also hidden. On the original Grand Cherokees, the exhausters were styled into what looked like a vertical vent on each rear pillar behind the rear side glass window and just before the tailgate. They are basically a rubber sheet or flap hung from the top on the inside. Then gravity keeps the rubber flap covering the hole, then when the inside pressure builds up, it opens the bottom part of the flap to let the air out of the interior so the blower can push more in. On the second generation Grand Cherokee they moved them and incorperated them into the tail lights. That is those "vent holes" in the side of the tail lights. On the 2nd generation Barracudas, they are built into the doors. The inlet is on the bottom interior of the door and the outlet is on the back of the door towards the bottom (if I remember correctly) you have to open the door to see it. I just tried to walk out to get some pictures, but there is too much snow between me and the cars and spare doors to get some pictures (and now there is snow inside my shoe and my feet are cold and getting wet).
My point of all of that is: Could your exhauster "flaps" have been removed and could be letting air in from there? Check to make sure that they are there and straight not peeling back (distorting) from age.
Maybe someone could get some pictures of theirs to post (who don't have to dredge through snow to take them).