Front windscreen washer sprays on back window?

The windscreen wipers on our 04 reg Megane have gone crazy. Suddenly, when you pull the washer stalk back for the front windscreen the wipers start moving, but the water sprays out of the back window. The rear window washer works fine.
The only fuse box I can find is tiny and located down the left hand side of the glove box, but have no idea which fuse it is because there doesn't seen to be enough of them!
How do we fix it???