In reply to robbiebrookie:
It does depend on where you go.
I've been 12 times in the last 10 years and seen evidence in car parks, but only once seen a car that had been broken into whilst we were there and it was just one out of 10 or so cars in the Canche Aux Merciers car park in August - I would say the height of summer (yes I've bouldered in font in august!) was much worse for seeing glass on the floor. (and it was just one out of 10 or so cars in the Canche Aux Merciers car park in June)
Parking at Barbizon is busy at weekends, and well populated during the week and seems quite safe (cafe) and there's enough at aprement to keep you busy for a month, but you will be 15-30min walk from the car. If you are really worried this would be the safest place, and you could park in the village in exchange for a longer walk in.
Trois pignions probably next best if you can get there early especially if you go on a Sat when the shoe man is there if you get a spot near him
Bas Cuvier is busy, but also the easiest smash and grab for a gang due to being on the N6 and lots of non climbers about and the climbs here are polished and dirty in places
at Elephant you can climb insight of the car park if you wish, and seems to have a lot of climbers milling about most of the time and feels pretty safe.
The smaller and more remote parking spots like Canche, 91.2, diplodocus etc. have always seemed fine, but they are of course very secluded and don't see as much traffic. At the really small parking spots I doubt theives really know they're there
The trick of course will be to make it look like you have nothing of value in the van (and/or take the valuable stuff with you)