With car windshields, the point the water drips down your window is not necessarily the spot that leaks. Leaks on the driver's side may originate on the passenger side, and vice versa. Fortunately, you can reseal many window leaks without knowing their exact source.
Modern car windows are bonded onto place, and you can use 3M Window Weld Resealant, applied with a caulking gun in a thin bead along the window's edge, to seal the entire window. Window Weld's is thin enough to flow and re-seal any nooks and crannies between the window and butyl or urethane glue used to seal leaks. Before applying the resealer, you should remove any decorative trim.
Leaks on older cars, which hold windows in place with a rubber gasket, can be resealed with 3M Grommet Windshield Sealer. It flows between the gasket and the window, and stays pliable in order to keep the window sealed.