I am trying to find a bonnet release cable for the sprinter on the internet, so far I haven't had much luck, the sites claiming to stock them don't.
Once the grill is removed; it pops out fairly easily; you can unclip the release catch for the bonnet. the model I have, 308, there is the usual manual release switch at the front, to the left of the central latch. to the right is the socket for the release cable activated from the switch in the cab. I have found this really simple to pop open with a screwdriver once you have found the exact spot. push the screwdriver in and lever it clockwise, pushing the handle of the screwdriver down toward the left, while holding the manual release catch open. the bonnet should just pop open.
snakeappletree, March 2009