moisture in trunk

I have a 99' Jetta new GL, I keep having moisture on the underside of the trunk mat, and in the driversides back seat. I have no clue where the water is coming from. The top side of the trunk mat is fine it isnt wet at all. Just under where the spare tire is. Has anyone else experince this problem and did you find a solution to the problem. Please some one help, I am putting subs in and need to get rid of the mosturie.