In a large bowl mix garlic, onion, soy sauce, golden syrup, apple juice, Worcestershire sauce, molasses, wine, Italian-style salad dressing, distilled white vinegar, garlic powder, salt, onion powder, Cajun seasoning, chilli flakes, seasoning salt and brown sugar.
Score pork shoulder 5mm deep. Place in the bowl with the marinade mixture. Marinate at least 4 hours in the refrigerator.
Preheat the barbecue for medium high heat and lightly oil grate.
Cook marinated pork shoulder on the barbecue 3 hours. Marinate frequently with the mixture while grilling.