Place chicken in a baking dish. Whisk olive oil, red wine vinegar, lime juice and garlic powder in a small measuring cup. Pour olive oil mixture over the chicken. Cover and place in the fridge to marinate overnight.
The next morning, place onions and bell peppers in the bottom of a slow cooker.
Sprinkle the powdered garlic and cumin over the vegetables.
Place the chicken on top of the vegetables and pour marinade over it all.
Pour the salsa evenly over the chicken.
Put the lid on, and cook on low for 7-8 hours until chicken is cooked through and fork tender.
Remove chicken and shred. Serve chicken with peppers and toppings.