Just a thought this, which will probably be rubbished by the well informed guys on here, (or indeed by you if your screen wash additive is deep blue!) but, my screen wash additive is lime yellow. I believe the pipe serving the back screen wash goes from the screen wash reservoir down the offside of the car, up the back wheel arch, side column and to the tailgate. Could be leaking. (I can hear the raspberries being blown now!)