just noticed you said you park facing downhill the leak could be coming in the truck at the back and traveling to the front also. i would just take a water hose or go to a car wash and run water all the seams on the passenger side until you see water coming in might take a while but works everytime to find the leak. if you were close by i would give you a hand

 

The problem on my pickup is my door seals. The other day it was snowing really hard and I had a pile of snow in my pickup on the door.

 

My 2010 Ram 2500 shortbed 4x4 had water in back seat passenger side after a hard rain. Took it to a dealer and he knew it was the third tail light above the back window. This light was removed and all water was removed and the light was coated with clear calk and installed. Whenever this light is removed to change a bulb it must be resealed. This dealer was instructed to reseal all incoming trucks as/per letter from Dodge. The first dealer I went to in Gulf Shores, AL said they have never heard of a water leak in a Dodge Ram.

 

^^^ yeah the 3rd light is the common issue when the carpet under the back seat is wet....

Dont let it mildew though. It will stink

 

the TSB that houseape posted up was the cause of this in my truck. I had a small leak that would come out of the grab handle only in monsoon like weather. I mean rain that you thought you needed to build an arc. Did exactly what the TSB said.. removed the weather stripping seal and put clear silicone along the seam and around each screw that was holding it on.. Been well over four months and havn't seen a drop come back yet