With the solid glass and manual sliders it's always the 3rd Brake light, with the power window it could be either. I'd say to take the screw out of the brake light and silicone the **** out of it and put it back on - and hope that's it because I don't think anyone has solved the power slider leak short of replacing it.
Symptoms are the same, when the brake light leaks it goes around the window and underneath it to the same place as when the window leaks...
Photo homage of my beloved old truck. Sold 8/2014
New ride: 2014 2500 CTD 4X4