I would recommend that you listen for the washer fluid pump for the rear washers when it is activated, if you don't hear the pump running, check for 12 volts at the connector when the rear washers are activated. If you have voltage, then the pump is bad. These pump failures are common on other cars, not sure about yours, but that is where I would start. If you hear the pump running, then look for a leaking fluid line, you may see fluid leaking from somewhere under the vehicle, but it could be leaking inside the car too!
I worked at the factory where this vechile was made. The most common problem if the motor is running and there is fluid in the resvior is a kinked hose on the rear wiper arm. This model only has one resivor for the front and rear both pumps are located on the tank itself.