Most wiper motors have two live feeds, one that comes from your switch to start the wiper going and a constant (ignition) live to make the motor return to the park position.
there can only be a few causes of the symptoms you have,
1 Blown fuse = Check all of your fuses ( not just the one for rear wiper)
2 Broken wire = Check the power feeds to the wiper motor with a meter
3 Faulty park sensor on the motor.
