1997 Plymouth Voyager cabin air filter

Our 1997 plymoth voyager has a foul spoiled milk smeill that is coming from the ventilation system. We were told it was mold and had it treated. However, the smell returns within a few days to weeks. We have used Lysol to treat the problem as well with no satisfactory results. We were told to check and see if the cabin air filter needed to be changed. That sounds good in theroy, except we can not find any information regarding a cabin air filter. Therefore, the question is could the odor be a cabin air filter? If so, where is it? How difficult will it be the change it ourselves? Where is the best place to get the filter and what part am I looking for. Finally, it this is not the problem, any ideas?