Water under carpeting in third row
 

About 8 months ago I noticed a strange "mildewy" smell in my Expedition. I discovered that my carpet was wet in the backseat. I pulled up the carpet and found that the carpet pad was soaked and it took many hours of blowing fans on it to dry it out. Since then, I can't seem to get the carpet pad to stay dry. I think it must be leaking somewhere, but I can't figure out where. We don't have a sunroof, so that can't be the source. Any suggestions?