check the rubber strip around the trunk that seals it when its closed. their may be cracks in it, or it have been removed alltogether. grab some gloves and lift up the carpet. under the spare tire there should be a peice of metal that can be removed. thats the drain. use it to get the water thats alreadfy in the trunk out. if the problem persists i would recomend removing the trunk carpet and leaving the drain out so the water will not build up in the trunk and make your carpet modly.