I have been having a problem with dampness underneath the rubber mat in my trunk. I do notice also that my right taillight is getting water inside it when I go to the car wash, even to the point where the waterjets broke my signal light (twice). (This happened when it was kind of cold out though). The right taillight piece also looks like it may be slightly separated from the body, or like it needs to be tightened or adjusted. Is this possible ?
Tonight I took everything out of the trunk including the mat, just to let things air out, and sprayed Lysol in there. Hopefully no mold or rust has formed.
It's likely. Pull the carpeted panels out and give it a good inspection...also check around the trunk seal by the rear glass to make sure water isn't pooling up and leaking through a small hole.
But if you have water in the tail light lenses...there's the weak spot(s). I took mine apart once to clean them out (dust) but didn't use the proper sealer during reassembly. Good luck!
Check out this thread http://www.foxtbirdcougarforums.com/showthread.php?34827-Ever-get-water-in-your-trunk-for-no-reason-SOLVED&highlight=water+in+truck
Pretty common problem.