can u hear the pump whirring when u switch the washer on? if not, pump / pump wiring is in need of looking at.
if you can hear it, look under the bonnet for the tubing that goes to the rear washer. it will have a valve on it somewhere - circular bit with a triangle on it. That is a non return valve. Remove the pipe from the input side of it. ( Input |> Ouput ) and then try the washer, it should squirt water out that hose. If it does, you have faulty non return valve. Halfords, motorfactors of fiat will sell them. Summit like 50p