In a large bowl, mix garlic, onion, soy sauce, golden syrup, apple juice, Worcestershire sauce, treacle, wine, Italian salad dressing, vinegar, garlic granules, salt, onion powder, Cajun seasoning, crushed chillies, seasoning salt and dark brown soft sugar.
Score pork shoulder 3 to 5mm deep. Place in the bowl with the marinade mixture. Marinate at least 4 hours in the refrigerator.
Preheat barbecue for medium high heat and lightly oil cooking grate.
Cook pork joint on prepared barbecue for 3 hours or until the internal temperature has reached a minimum of 75 degrees C. Marinate frequently with the mixture while grilling.