putting rainx or any other product will be a waist of money and your time dude, from the serverity of your problem im pretty sure the sound proofmateriel/foam is proberly very wet under the carpet, and though the carpet may not feel very damp its all underneath so you may have to lift the carpets fully or just enough to allow the moisture to escape from underneath, check your spare wheel well aswell to see if theres a pool in there as they are prone to collecting due to the drainage holes blocking up, you need to get a heater in there whilst the carpets are lifted to fully dry out the underneath.