Place steak in plastic bag or non-metal baking dish. In small bowl, combine marinade ingredients. Pour over steak, turning to coat. Seal bag or cover dish; marinate at least 6 hours or overnight in refrigerator, turning once or twice. When ready to barbecue, drain meat, reserving marinade by placing in small saucepan. Add steak sauce to marinade; blend well. Heat on grill. Place steak 4 to 6 inches from medium-hot coals. Cook 30 to 40 minutes, turning once, or until desired doneness, brushing occasionally with marinade. Meanwhile, heat foil-wrapped tortillas on grill until thoroughly heated and steaming, wrap in cloth napkin or towel to keep warm. To serve, cut steak across grain into thin slices. Spoon any remaining marinade over slices. Arrange steak, warmed tortillas, onions, chilies, butter, salsa and guacamole on a large platter. Spread butter on tortilla; top with meat and any combination of vegetables or sauce. Roll up to eat. Note: Be sure to heat basting sauce thoroughly to ensure safety for use as a sauce at the table.