A small leak in a car window has the ability to turn into a big problem. You're lucky if you catch the leak early, before major water damage. In the early stages of a car window leak, you may notice water on the dashboard, floors or seats. A musky, mildew-like smell can also be a warning sign that you have a window leak. Once you identify the problem, fixing a leaky window is easy.
Locate the source of the leak. If the leak is small, the easiest way to locate the leak is to spray a garden hose along the window seam and see where water seeps through. You may need another person to spray the window while another person sits inside the vehicle looking for the leak.
Remove the rubber that runs along the edge of the window by slowly prying it from the shell (or channel) of the window. Inspect the channel for missing caulk, cracks or rust. Also check to see if there are any cracks in the glass window itself.
Decide your next course of action. If you found missing caulk, wet or gooey caulk or a crack in the channel, then you can re-caulk the window channel yourself. If you see rust, then repairing it may be very complicated, as you'd need to repair and repaint the rusted channel. It is advised to hire a trained professional to do this repair for you. Similarly, if you see a crack in the glass itself, there is not much you can do besides have the glass panel replaced by a professional.
Re-caulk the window. After removing the rubber lining, clean out the window channel as best you can. Use lighter fluid (naptha) or paint thinner (white spirit) to assist you in the cleaning of these dirty areas. Insert a polyurethane sealant into a caulking gun and open the tube of caulk with a small hole so the tip of the applicator can fit into the window channel. Apply the caulk into the channel all the way around the window. Re-install the rubber trim before the caulk cures. A 2-by-4 piece of wood can help you press it in. Duct tape can be used to hold the rubber strip down while the caulk cures. Do not get your car wet for a few days until the caulk is completely dried.