Expose the window motor connector and unplug it. Connect a voltmeter (or 12V test light) and try the switches. If you have a digital meter you should notice the polarity reverse as the switches are alternated between the "up" and "down" directions. If the rear door switch does nothing, try the master on the driver's door.
If you do get power, start looking at the motor. Before completely condemning the regulator motor, reconnect the power connector to it and back-probe the wiring to see if voltage applied is correct with the load (motor) in the circuit. If not, the switch contacts are suspect. If so, the motor is suspect.
If you get nothing while actuating the switches with the motor disconnected, break out the diagram and start tracing.