In a slow cooker, combine tomato sauce, chili powder, cumin, salt, paprika, black pepper, garlic, and onion.
Stir until well mixed.
Add chicken breasts and coat them thoroughly in the sauce.
Cover and cook on low for 4–6 hours or high for 2–3 hours.
Remove the chicken and let it cool slightly.
Cut or shred into bite-sized pieces.
Return chicken to the slow cooker and stir well with the sauce.
Cover and cook on low for an additional 15 minutes.
Serve these Slow Cooker Chicken Tacos in warm tortillas with your favorite toppings.