Two possible and common sources of water entering the cab are: (1) rear window seal and frame and (2) bed floor and side seam above the area. It is very difficult to pinpoint the exact spots for both and requires a lot of patience. First, inspect these locations and check for any signs of visible rust stains. Next, have a partner slowly dribble water over the bed seam first before proceeding to the window; it is important to start at the lowest spots first since water always seeks and settles in the lower locations. Once you identify the spot where water appears in the cab, continue looking horizontally because water will travel horizontally until it eventually enters the cab. This is due to water being captured behind anything that hinders its path to the drip location.
Come back and tell us what you find, then someone will provide you with the best means to seal off the leak.