Few things can be more frustrating than trying to track down a leak in your car.
Sometimes it's easy to see where the leak is coming from, but often the source is a long way from where the water appears in the trunk or the passenger compartment. Here are some ways to find out:
The spray test. Water is like electrical current. It takes the path of least resistance. So you want to spray all the possible entrances. If water in the passenger compartment is the problem, spray all around the door edges, window edges, and so on. But the trick is to spray in an organized way. Don't just inundate the car with water until water builds up inside. That won't tell you anything.
Start with a door seam. Start from the bottom. Spray directly into the seam and move very slowly upward. You might take a minute or longer just to spray one door seam. The reason is that somebody will be inside the passenger compartment watching. When water starts entering, you'll know exactly where it's getting in from outside the car. If you sprayed the entire seam indiscriminately, you would not be able to pinpoint the leak.
A common cause of leaks around doors is broken, cracked, chipped or missing weatherstripping. If the weatherstripping is merely loose, regluing may solve the problem. If not, replace it.
The light test. After it is dark, get inside the car and have a friend direct a flashlight around the various seams, grommets, moldings and so on where water may enter. If you can see light through a seam, that is a possible leak area.
Talcum powder test. Rub a smooth layer of powder on the weatherstripping of the trunk lid or door, then shut it. Open it. There should be talcum powder deposited on the metal that the weatherstripping butts against. If there is a place, or places, where no powder appears, this may be a leak area.
Visual check. Sometimes a leak can be spotted this way. Simply eyeball every place that water is likely to enter. Check the weatherstripping with your fingers. Is it loose? If so, reattach or replace it.
Sometimes leaks can come in around the edge of windshields where molding has pulled away. Silicone any gaps that look wide enough to cause a leak. Silicone can also be used in areas where grommets don't fit as well as they used to. Silicone comes in tubes and is sold in auto parts stores. Silicone is easy to apply. Just squeeze it out of the tube and use your finger to plug holes or cracks. If the surface is clean and dry it will stick.
Weatherstripping cement also can be purchased at auto supply stores.