[Silver Supra]

I did not see this thread until it was bumped today, since I recently joined.
After reading all the posts, I suspect the car had standing water of some sort before purchase (could have been an inadvertent spill or other) that was left long enough to grow a bit of mildew and mold.
There may, or may not, be an obvious leak, since it may just be high humidity that is enough to activate the smell again (usually higher when it rains). I have had this problem in a used car, and had to really search to find the mold/mildew, and then had to use good cleaners to get rid of it. It usually is found in a spot that has at least one absorbent surface - like carpet.
If there is mold, all the other measures mentioned will not get rid of it, even if they lessen the effects.