I remember reading about this about a year and a half ago and it ended up being water intrusion/leak. Pull the mat out of the back and check under the Styrofoam for water. There are several places where the water can come in, the gas cap seal, spoiler seal, and if it is in the front carpeting I would check the front drain to see if it is clogged and check and change your cabin air filter.. Also the is a guy in Brandon/ Riverview, Fl on Ebay that may have a replacement carpet. Nickoz is the store name.

Good luck on your search.