1. Heat a heavy skillet over medium heat. Place the dried guajillo chiles into the skillet and cook, turning occasionally, until the color changes and the chilis have puffed, about 5 minutes.
2. Set the chiles aside to cool for a moment. Meanwhile, toast the cumin and cloves in the hot skillet until the cumin seeds begin to pop; remove from the pan and set aside. Remove and discard the stems, seeds, and veins of the chiles, and place into a small bowl.
3. Pour the boiling water over top, and cover. Let the chiles soak for 1 hour.
4. Grind the toasted cumin and cloves into a powder, and place into a blender.
5. Add the ancho chile powder, onion, garlic, oregano, thyme, vinegar, and lime juice. Remove the chiles from the soaking water, and place into the blender along with the 1/3 cup of tequila. Cover, and blend until a smooth paste forms.
6. Place the beef roast into a mixing bowl, and pour the guajillo chile past over top. Coat the roast on all sides with the paste, then cover the bowl with plastic wrap. Marinate in the refrigerator overnight.
7. Transfer the roast and marinade to the crock-pot. Cover and slow cook on high for 6 hours. The meat can be left in the crock-pot in the keep warm function for 4 hours if needed.
