Driving in rain storm with a leaking windshield is one of the more annoying things that a car can do to bother the driver and passengers. A bad leak can soak clothing and the car's carpeting very quickly. The problem is that the leak is easy to forget between rains. Sometimes you even try to convince yourself that it will not happen next time. Once the leak starts, it will rarely heal itself. You have to either repair it yourself or have it repaired.
To begin the repair, find the place where the leak starts.
Like a leaking ceiling at your house, the drip is not always under where the leak really is. Water can come in several inches away from where it drips into the car. A close inspection of the windshield in the general area of the leak may yield the actual spot where water can get into your car. However, you cannot always locate the place simply by looking. Get a bucket of water and something to dip it with to pour on the windshield. If you can have someone sit in the car and watch, it will work even better.
If the leak is not obvious, you will have to use water to find it.
Block of your windshield into about 2 or 3 inch section where you believe the leak might occur. Pour water over the first section and see if it finds its way inside the car. If yes, mark that spot and keep repeating the test until you have either identified a few areas to be sealed or realized that you have only one small area to fix. Mark all leaking areas. Most windshield leaks will be fairly small.
Buy material or a kit to seal the windshield leak.
Auto parts stores and some discount stores sell what you need to fix this leak. There are kits available with various toys to apply the sealant and cover the spot while the sealant dries. However, it you have a pretty good feel for how to apply a decent clear silicon sealant without getting it all over your car and windshield, it may be a much cheaper way to go.
Do not be overly generous with the sealant.
You want to apply enough sealant to stop the leak, but not enough to leave large amounts of residue on your paint and windshield. Try to force the sealant down into the crack where the windshield leaks. Continue to add sealant until the crack is filled. This will take surprisingly little if the opening is not too large. For a really big area, you may want to go to a professional. If you have recently had the windshield replaced, you may want to choose that route anyway.
Give the silicon or other sealant time to harden before subjecting it to rain or water.
The instructions with the sealant will recommend how long to let it set up before allowing water to touch it. Most of the commercial sealants need about 24 hours. If you expect rain, get some duct tape and attach some type of plastic barrier over the repair to allow the sealant time to dry adequately.
Test your work for leaks.
If there is no rain in the forecast after the sealant has dried, test your work with the bucket of water again. If it does not leak, pat yourself on the back. If it does leak, repeat the procedure for repairing it. More than likely, you just need to extend your repair a little farther on one side to complete the fix.