If you can find a hole where rain leaks in, just fill it up with Shoe Goo, a glue like substance in a tube designed for shoe repair. I got my Shoe Goo at K Mart a few years ago. I guess, they still have it.
Step2
For hard to find leaks, run your vehicle through a car wash with you in it, and try to determine where rain is coming in.
Step3
If a leak is entering between the top of your wind shield and the frame of your auto, you probably should apply Shoe Goo across the entire top of the wind shield.
Tips & Warnings
You can repair holes or cracks in a wind shield with Shoe Goo, but it's not transparent.
Another cause of leak may be a door, that is not properly aligned when shut.
If your lucky, you can readjust the door hinges for a snugger fit.