If you have a leak where the glass is inside the rubber sheath around the pillar, take 3M black weatherstrip and place the tube's tip in between the glass and rubber. Squeeze the tube and over fill, so a bead will be squeezed out as you work your way along the rubber. Whatever squishes out, clean quickly with small artist painter's trowel. What small film remains, use a cloth and brake clean to remove the rest. Use the brake clean sparingly.
Want to pinpoint were the leak starts? Take a garden hose and have someone sit inside the car. Run the hose without any fittings on the end, and turn the water pressure on lightly. Start from the bottom of the windshield and work your way up. When the eyes in the car visually sees where water is coming in, you have your starting place. As you travel higher along the windshield's rubber with the hose, they might see just how much or little the length of the leak is.