2002 Mercury Sable GS- Water in sparetire area

Hi.I have a 2002 Mercury Sable GS sedan with a water leak problem in the trunk area. My inside passenger space has been smelling a little musty. I checked all the floor and seats and did not find any wetness. I then emptied the trunk contents. I noticed a little wetness on the carpet piece near the trunk lock area. So I lifted the carpet there to expose the spare tire area. I was shocked to see my spare tire covered with water. There was a lot of water! I have no idea how long this had been happening. I removed the spare tire and all the hardware there too. I saw the rubber plug on the floor of the trunk and pulled it out. All the water emptied out. Now my question is....what could be causing this water to collect and how do I fix it? Also..there is mildew and mold growing on the back of the hardboard cover to the carpet piece I removed. How is the best way to treat this or should I try to buy another cover? I appreciate any help and suggestions! thanks!

My '87 did the same. It was the weather stripping around the trunk.