Under carped in back is Soaked ?!

I have only had this vehicle about 3 months (95 4.3 with only 33K, previous/orignal owner had many cars and rarely drove)

there has been something in the back that seems to roll all the way from one side of the car to the other, something small and when i pulled off the plastic runner under the tailgate put my hand under the carpet, and the foam is SOAKED all the way back as far as i can reach

sure its raining today but it looks like it has taken more than one rain to get this wet.

are their any plugs underneath the car ie. water coming from road, or could it be leaking somewhere along the glass or tailgate somehow, there are no watermarks showing leakage

Thanks