To servefine chopped sweet onion, chopped cilantro, avocado or guacamole, cotija cheese and lime wedges for squeezing.
In a food processor, chop the garlic cloves. Add the chile powders, cumin, black pepper, vinegar, salt and 1/2 cup water until blended.
Place the meat in a large crock pot. With a spatula, scrape the chile mixture out of the food processor and spread it on the top of the meat. Add 1/4 cup of water to the bottom of the crock pot, cover and cook on low for 8 hours or until meat is tender. Carefully remove meat with a slotted spoon onto a large cutting board. Shred the meat with two forks.
Degrease the cooking broth and boil it down in a saucepan to concentrate the flavors. Ladle it over the meat and then build your tacos.