I know there's an option of a sort of flap that opens on the roof at the back, for carry a ladder or anything longer than the van. The Renault Extra van had this, and the Renault 4.
Didn't know it came electrically operated, if this is the vent you mean.
Just in case anyone was following this, I fitted a Flettner 2000 rotating roof vent to my 1.3 Doblo Cargo yesterday (www.flettner.co.uk)
Fitting required a 95mm. hole to be cut in the roof - a steady hand and an even steadier nerve was the order of the day. Still the vent does its job and the van interior no longer smells quite so stale - I have three working dogs who very often end up wet so this mod was a very definite neccesity.