Just because you see water leaking from the upper-left corner of your windshield doesn't necessarly mean that's where the water's coming in. Many leaks on the driver's side originate on the passenger side, and vice versa.

Fortunately, you can reseal many window leaks without knowing their exact source. For modern windows that are bonded onto place, use 3M Window Weld Resealant. Applied with a caulking gun, Window Weld's thin viscosity allows it to flow into nooks and crannies between the window and butyl or utethane glue to seal leaks. To apply, remove any decorative trim and squeeze a bead of the resealer around suspected leak areas.

To reseal leaks on older cars where windows are held in place with a rubber gasket, use 3M Grommet Windshield Sealer. It flows between the gasket and the window and remains pliable in order to keep the window sealed.