Drop the regulator assembly into the door first. Then fit the vent window.
Attach the regulator assembly loosely. Roll the glass onto the bottom track rollers. Then install the rear track. Then tighten up the regulator.
Finally adjust the rear track so the window rolls up and down without binding or excessive free play. The first door will take three hours. The second about half a hour. The learning curve is fast.
Hope this helps!
John Twist
Safety Fast!