I always use soap ,( household soap for dishes) this will helps better than WD 40 , and slide de window horizontal .
Mount rubber 1/2 thick into the frame , and the glass 1/2 thick into the rubber on the vertical frame rail, and than hammered this directly on the glass forward.
And when the glass is on it's place mount the vertical rail , always scary to get scratches on the door and rail trim.
the trim on the top of the door has a small space to fit the horicontal rail
Use a lot household soap for dish , to soak the rubber more greasier than WD40
Have done several, never a problem .
Simon