Combine the brown sugar, honey, El Pato, apple cider vinegar, soy sauce, and garlic in a medium bowl.
Add the lil' smokies to a slow cooker (you can pour the juices from the package in too). Pour the sauce over the sausages and stir to coat.
Cover the slow cooker and cook on HIGH for 2-3 hours.
Give everything a stir before serving. You can keep the smokies in the slow cooker on WARM or transfer them to a serving bowl and top with chopped parsley, if desired.
Notes
If you can't find El Pato (yellow can) in the Mexican section of your grocery store, you can use 1 cup of regular tomato sauce with red pepper flakes to taste. If you don't want the heat, use regular tomato sauce with no pepper flakes added.