Jfenn.

Take one of those gallon juggs and pour some water
on each bottom corner of the windshield on the out side
of the glass.

The rubber gasket around the glass dries and shrinks
over time, this will allow water to run in between the
glass and the gasket.

After pouring water on the corners, look inside the
car, in the same area if you have water coming in
you found the cause.

The best and only way to fix is to replace the rubber.

Louis.