Years ago I had a Cavalier and it used to mist up when the blower was on. I discovered that in the engine compartment there were drainage holes - these let the water that comes off the windscreen drain through the engine compartment. Unfortunately, years of parking outside had meant that these had become blocked and when it rained, this area of the bulkhead filled up with water. This wouldn't normally be a problem, but in the Cavalier, the air intake which feeds the windscreen and the internal heating system is positioned in that area of the bulkhead .... so damp etc got into the car and misted the windows up.

Once the drainage holes had been cleared the problem went away.