Set the slow cooker to slow.
In a small bowl, mix together the brown sugar, smoked paprika, fennel seeds, crushed garlic and chilli powder if using. Add 1 tsp salt and lots of freshly ground black pepper.Rub this spice mixture all over the pork.
Place the pork in the slow cooker and pour the apple juice around the joint. Place the lid on top and cook for a minimum of 8 hours. It would do no harm at all to leave it cooking even longer - I've often left mine in the slow cooker for up to 12 hours.
Remove the pork from the slow cooker and when it's cool enough to handle, remove the thick layer of fat and shred the meat with a fork. It should fall apart at the merest touch.
If you're planning to re-heat the pork, don't forget to save the meat juices.