I got to mine by removing the gearlever gaiter (boot) and removing the metal plate under that. It still wasn't easy but you can disconnect the plug connecting the switch to the loom and see if it's failed, it should be open circuit when not in reverse, or removed.
Just to add, the switch has a lock nut which determines the depth that the switch sits inside the box. if you remove the switch then take care to keep the lock nut in the same position. To set it up, should you of disturbed it then i put the box in reverse (ignition on), had the missus stand at the rear of the car while i screwed the switch in. Once the reversing lights came on then i gave it another full turn and nipped the lock nut up. I'm not entirely sure all switches are the same so a full turn on mine may not be enough or too much for yours. trial and error i reckon!
