I found the small crack in the sealer under the back glass on the right side. Just forced some fresh sealer in the area and my problem was solved.

 

Just fixed this same problem in my 04 dakota. On mine it was where the heater core was bolted to the firewall. The foam seal sound the firewall wasn't stopping water. I took it to my mechanic, they found it and put jb weld around both the inside of the cab and and I the firewall where it is bolted to it. Had this fixed about a month or so ago and haven't had a problem since.

 

I did the elbow on the drain to fix this in my Dak