Those things usually get goofed up by body shops, glass installers or amateur upholsterers. The most vulnerable parts of the rear window washer system are the washer nozzle, which gets clogged with paint, polish and grunge, and the tube that runs from the washer tank to the nozzle and often gets pinched or sliced in two.
First, make sure your pump is working. Then check to see that the hole in the washer nozzle is clear and try blowing compressed air through the tube from the direction of the washer tank. If air doesn't blow freely from the nozzle, you'll have to pull up some carpet and interior panels to make sure the tube isn't pinched or cut. Don't blame me, my friend, I didn't design this weird hose routing system.