car?

In front of the windscreen is a grille. Under this is an area where water can collect.
In each corner somewhere there should be a drain tube to allow the water to escape.
The tubes can get blocked with mud and leaves ect and water builds up and can get into car.
You may have to remove wiper arms to get under this panel.
Check this first.
If you can pull carpet up on passenger side and get someone to spray hosepipe over windsreen the water will run to that area.
You stay in car with a torch and see if you can see if any water comes in.

dave
February 2011