Score your shoulder roast with a skewer or sharp knife to allow the marinade to penetrate deeper.
Place the scored pork shoulder into the ceramic bowl of your slow cooker. In a 2-to-1 ratio of vinegar and water, pour enough vinegar and water into the slow cooker to assure the pork is completely covered. Add the sea salt.
Put the ceramic bowl in the refrigerator and allow the pork to marinate 12–24 hours.
After marinating, pour out enough of the liquid from the ceramic bowl to expose about 1/2-inch of pork.
Add the onion wedges to the remaining liquid.
Season the exposed surface of the pork shoulder with the cumin, mustard, chili powder, cocoa and sugar.
Place the bowl into the base of the slow cooker and cook on High until the pork is tender and falls apart easily, 6 to 8 hours.
Carefully remove the pork to a cutting board and, using a pair of forks, shred the meat. Remove and discard any excess marinade or fat.