I am not sure if Challengers had insulation form the factory or not, but it couldn't hurt to add it. If it helps, I have a '74 Charger SE that had the jute-insulated side facing out to the trunk, from the factory. The insulation was visible when you opened the trunk. When I restored the car, I replaced the old board and faced the insulation side towards the rear seat back. That way, the trunk looked cleaner and the jute padding was less likely to get damaged or collect dirt. Not sure which way would be correct, but I would face the cardboard side towards the trunk.