It is most likely to be limescale blocking the jet on the wiper arm. I have the same problem with mine, it can be clearly seen. Apparently, the "jet" piece itself can be pulled out of the arm. If you can get it out, soaking in hot water or vinegar may break down the limescale or the jet can be just be replaced.
There is also one way valve in the pipe run just in front of the rear bench seat (to the right hand side when looking at the seat). It is behind the carpet, to get to it, you need to remove the bench, this may be blocked although mine wasn't.
There is also another join in the pipe at the rear tailgate. To find the join, open the tailgate and look at the left hand side beside the hinge. You will see a rubber pipe which clips into the body. Remove it by pulling and you will see the pipe connector. Split the pipe and see if water spurts out when you use the washer switch, this will prove that the problem is between there and the wiper jet if it does. One other point, make sure your water is topped up as the pipe for the rear washer appears to join the reservoir higher up than the pipe for the windscreen washers.
