If I may-
Look at your tailights, and follow the body fascia around towards the front,(either side of the car) and you can see a slotted vent. The other side of this vent is inside your trunk area. There is a rubber flapper in between the outside air, and the inside air. Inside your trunk, in the right and left rear corners, there is some mesh netting, covering the trunk side of the vent. (or at least there is, on my '01)
Make sure you don't have any junk stuffed into the corners of your trunk, blocking the air flow.