Happened twice heavy garden hose stream and pressure wash.

 

I had a 2001 Mustang convertible that leaked all kinds of water. The water would come in around the top somewhere and leak into the area the top dropped into behind the seat. Then it would roll down into the trunk and sometimes the rear floor boards. I tried for 3 years to figure out exactly where the water was coming in around that stupid canvas top. Eventually, I just gave up and learned to live with it. I drilled drain holes in the spare tire well so the water could escape and I just threw out the trunk liner. I just made sure anything important in the trunk sat right on top of the spare and it stayed dry. The rest of the trunk would get all sloshy. Then I would drive around hoping someone would crash into it so I could get rid of the POS.

Eventually, I just traded it in for my Ram. I do not miss that car. Not even a little bit. :-)

 

Similar problem on my Gen 2 was the CHMSL. Just had to remove it and run a bead of silicone around the base.

 

My first place to check would be the 3rd light as others have said. I had it happen to me also when I changed it out to an aftermarket light.

 

I have an 08 quad cab ,and i noticed a leak at the top/middle of my window.The dealer first replaced my third brake light completly, when that didnt work ,they took out and reset /sealed the rear window. no more water so i guess that was it.

 

Have had my new Ram 3 weeks now. After 21 days of rain here in Tampa, Fl. I found the storage under rear seat FULL of water. I have the rear window but found the leak coming in thru the third brake light. Took it out and siliconed the seal, hope that takes care of it before the next rain.