Those types windows tend to leak because the "rubber" or flexible parts shrink and grow hard over time. In that condition they lose their ability to seal against water.
Sadly repair parts are almost non-existent and in most cases the entire frame would need to be removed, disassembled, repaired then re-installed.
Equally sad these types windows were notoriously cheaply made, getting more than 10 years life out of them generally isn't too common especially if the vehicle is not stored out of the weather and sun when its not in use.
Sorry to be such a downer but being in the auto/truck glass business I've had to deliver this sort of bad news to more than a few.