two potential causes one cheap to fix, one not

1) cheap mold and mildew in the ducts and drain pan causing foul odor. Solution is to flush with water containing a few tablespoons of bleach (chlorine) in a gallon of warm to hot water. Pour it in the air intake slowly (like over 2-3 minutes) with the fan running. Leave it for a couple of hours, then flush with fresh water just the same. You have to kill the mold and mildew, not just cover it up.

2) If the smell is 'sweet' and stronger when you turn on the defroster, you have a small antifreeze leak. Fix is remove and replace the defective heater core. (15-20 hrs + $200 for new part) unless you are in warranty coverage somehow.