Brake Light problems of the 2009 Dodge Ram 1500

Woke up to a rain filled day. I went out to my truck and entered the vehicle. I looked in the rear before backing up. I noticed that my head liner was soaking wet on both corners of the c post. I put my truck into park and checked the rear of my cab. Not only was my roof liner wet but, the rear seat and floor was fully wet. The water kept coming in throughout the day even when I was driving. The next day I got it checked. I found out the third brake light seal was not working properly. This has now happened to me 5 times in the last 2 years. I have only had this truck for 2yrs 3 months. I am unaware if my seats or carpet have mold or mildew do to this water damage. Please help as I do not want to have a rust or mold in my truck.