I have had the same issue and its driving me nuts. Rear washer not working. New pump fitted. Not working. Checked old pump on a spare battery and its fine. Should have tried that first but hey ho. If I swap over the plugs on the pumps, the windscreen switch then works the rear pump. So ordered a new dash switch for rear washer. Still not working...
To access yours. Take off the nearside front wheel. Remove the plastic panel to get access into the rear of the front bumper, and washer bottle with both pumps is right there. You might have a problem removing the three Philips head screws on the inside turn of the bumper. Mine were rusted in solid.