The cabin filter is behind the glove box and is not that difficult to replace. It is also possible that your heating system may have some mold or mildew inside. I have seen that cause very offensive odors as well. There are some sprays that will kill the mold/mildew and can be purchased at a good Auto Parts Store. I would not just spray in bleach, because that can get on to your seats and leave big white spots!
The cabin filter has been left in there far to long and now has a life of its own ,its rotten and is filling your car up with bacteria either get a cabin filter spray or replace the filter either way it cleaning out !