Once you have removed the car's interior door panel, look at the window mechanism. Look carefully at the window mechanism, is the window off track? If the car window is off track, usually it is because of the rollers have become unclipped from the regulator arms, or they could be damaged and worn out. So in order to get the window mechanism to function correctly again, you will need to clip back the rollers, if they have become loose, or replace the roller clips if they are damaged or faulty.
If the rollers in the window mechanism are fine, then this problem is caused because of the motor being faulty. So i suggest you replace the motor and hopefully the car window fill no longer fall down into the car door.