my friend had this same problem we couldn't figure it out till we jacked the car up and found a burnt plastic bag wraped around the exhaust .

 

hey the smell came back very faintly in the rain. I don't think I had a bag underneath my car, but I did turn on the rear window and front window defroster... I didn't actually turn the air on, just the **** and button to on.
I had the AC on for hours a few days ago and there was no smell.

 

Maybe the heat elements of the rear window defroster are burning something, such as the window tint.

 

but the smell is coming through the vents?
does the blower motor actually turn on when the *** is switched to defrost (front)

 

That wouldn't fit if the smell is coming from the vents. When you are in defrost mode alone with fan OFF, the blower obviously doesn't run, but I think that recirculation is turned OFF and air comes from the outside.