In a large bowl, mix garlic, onion, soy sauce, corn syrup, apple juice, Worcestershire sauce, molasses, wine, Italian-style salad dressing, distilled white vinegar, garlic powder, salt, onion powder, Cajun seasoning, crushed red pepper, seasoning salt and brown sugar.
Score pork shoulder 1/8 to 1/4 inch deep. Place in the bowl with the marinade mixture. Marinate at least 4 hours in the refrigerator.
Preheat an outdoor grill for medium high heat and lightly oil grate.
Cook marinated pork shoulder on the prepared grill 3 hours, or until the internal temperature has reach a minimum of 165 degrees F (75 degrees C). Marinate frequently with the mixture while grilling.
