My mechanic thinks the problem is leaves getting into the lip around the top of the trunk. I have tried to remember to clean it out before it rains.
I just looked at the trunk and there is no water around the spare tire, nor does the carpet or anything smell around there.
Any other thoughts? Could the must/mold from previous events have gotten into the a/c system?
Are you able to answer my follow-up question?
I can't see the smell getting into the AC from a water leak event. What happens to the AC is that pollen, seeds, dust and such builds up on the evaporator core - the device that make the air cold. If you add moisture - which happens because the air loses its moisture on the way into the car - cool the air, the moisture comes out. So, you've got the makings for mold / mildew on the evaporator core.
When you turn the AC on, you get a blast of that moldy smell.
I know that General Motors markets a product that is designed to make the air conditioning smell better, and I'm sure there would be something available in the aftermarket as well.
What is an aftermarket? And I don't want to use chemicals....Are you aware of this problem with other 2004 Subarus?