Could be a break in the wiring just before it goes into the 'grommet' into the boot. Although they may look fine, the insulation is flexible, but the wiring inside isn't, so could be a break that you can't see. Have you got, or know someone with, a multimeter? If theres no voltage going to the motor, that could be the problem. The the relay for rear wiper should be behind the trim under the steering wheel, above the accelerator pedal, 'big one, bottom right'. If there is voltage, check the motor mech itself, Dis-connect lever from spinle mech inside boot lid and see if you can turn the spindle by grabbing the wiper arm and moving back and forth.
Does it make any form of noise? Clicking etc?