Place the onion and garlic in the slow cooker.
Trim the visible fat off the pork shoulder, and cut it into a few large pieces if needed to fit in your slow cooker.
In a small bowl, stir together the paprika, chili powder, cumin, and salt. Rub these dry seasonings on the pork. Place the pork in the slow cooker.
In a medium bowl, stir together the vinegar, tomato sauce, and maple syrup. Pour the sauce over the pork.
Cook on low for 8 hours. The pork will be cooked through after just a couple hours, but you want to keep cooking it "low and slow" so that it absorbs the flavors and shreds easily.
After 8 hours, shred the pork by breaking it apart into small pieces with two forks.
Puree the remaining sauce and pour a little bit over each serving.