Had the car in bits today trying to find out why the water squirty didnt work on the rear of my Signum (Never has since I bought it).
Started off by pulling the rear hose of the high level brake light washers and tried the pump but nothing came out.
Then pulled off the 2 pipes from the washer motor and tried the pump again. This time water squirted out of both pipes coming out the motor itself.
So I now knew it was a blocked pipe somewhere. Started at the front of the car and worked my way along the pipe looking for a crushed pipe or something (I had no water in the car so knew it wasn't a holed pipe). No crushed pipe all the way upto where it goes in the rear quarter.
Pulled the pipe out of there to find a join that was infact a oneway valve. The oneway valve is just a bit of rubber that opens when water is pressurised at it (Pump running) but closes when there is no pressure (Stops the water running all the way back). My oneway valve had somehow managed to roll itself up and block it solid. I pused out the rubber valve/blockage but the rubber valve had a split in it so I have left it out for now.
So for anyone else with a rear water squirty that doesn't work but the motor works and the washer jet is clear that may also be the problem.
Water pipe at high level brake light:
Join/oneway valve in the nearside rear quarter behind panel:
And a close up of where it is:
And the oneway valve with the rubber/valve pushed out: