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Ok, The rear washer has a one way valve that sometimes can stick , it is in the tailgate of the vehicle and would involve removing the inner trim to inspect it and free it up. It can also be clogged at the nozzle or any where along the hose route, the best plan of attack is to start at the tailgate, removing the trim and looking at the valve and nozzle and hose in the tailgate since that is the most common area for problems.
I would also consider letting the dealer take on such a potentially involved task since your car is still under warranty.