Does anyone else have a problem with the rear washer corroding and blocking up? Any suggestions for a permanent solution?

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A design flaw with the wash wipe means that it tends to get blocked.  I've used a small piece of wire to unblock both jets, lift up the flap and remove the jet nozzle, ensure it's your nozzles that are blocked by removing the jet nozzle housing and use the wash wipe otherwise you could have leaks within the system.

New nozzles are pretty cheap.