The problem is there is no easy way to know exactly where the water is coming in at. In your case, it's most likely the top of the window. Then the water is running under the gasket around to the bottom then leaking onto your floor. When you are driving, it's the wind that forces the water into the gasket causing the leaks. You won't be able to duplicate that with a hose. To repair it without removing the windshield and gasket, do the following.
1. Pull out the interlock. That's the 1/2" wide strip in the middle of the gasket. This may need to be replaced depending how brittle it's become. Replace it with black rubber, not the fake chrome. It looks good and stays pliable forever. Cost around $15 either way.
2. Buy some windshield urethane from a glass shop. NOT silicone or anything else. Probably will use 2 tubes. It's expensive...
3. Mask off the body around the gasket with good tape.
4. Lift up the gasket as you go & run urethane between the gasket and body all the way around. Enough that it mooshes out onto the tape.
5. When completed, wet your finger constantly with glass cleaner (works best) and remove the excess urethane from off the tape & gasket. Smooth the bead out pretty as you go. Do not remove the tape yet.
6. Tape off the windshield inside the gasket and do the same thing between the gasket and the glass. Water leaks there just as common. (Be careful, the edge of the glass is the weakest part and too much pressure on the edge will send a crack flyin) Do not remove the tape yet.
7. Next, re-install the interlock rubber piece. This is done with a special tool. It's a pain without. Trick is to keep the gasket interlock channel wet. Spray glass cleaner works best! As you do this more urethane will probably be squeezed out from the pressure.
8. Clean up any excess urethane with above method.
9. Gently remove all the tape.
10. Any mistakes with the urethane will need cleaned up now with a special solvent. Nothing around the house will remove it. Only smear it.
11. Let the urethane dry for several days. No car washes or rain storms. You must keep a window down at least a couple inches and don't slam your doors. The air pressure will blow your leaky hole (heehee) open again.
This is what a glass shop would do. There are no gaurantees. It may still leak afterwards.
If you want to install a new gasket, replace the glass too. Don't plan on re-using it. Chances are it will break coming out. Don't everybody argue. There are many variables.
Best method: Drive it to a glass shop and walk across the street for lunch while they do it. My .02 cents. Good luck!
