Lower left corner of the "door chassis", with the door opened, there is a black push button. When you open the doror, the button get "released" and give the "message" to the window to go down (*).
Probably is oxidised.
First try moving a little bit the screw, if not unscrew it and clean it with a spray circuit board cleaner, it should work.
This should cover my poor English
(*) I am referring to items 19 and 20
I will try this approach as well. 
