I've got a 2006 Nissan Pathfinder SE 4WD with the potential problem of coolant and transmission fluid mixing in the cooler in the bottom of the radiator. I was wondering if I can get away with just bypassing the radiator cooler and adding a Hayden 679 external cooler. I feel like it would keep it cold enough I was just wondering if it would over cool it. I live in NC and sometimes it gets below freezing in the winter and I don't want to over cool it if it's possible. If I got a derale Transmission thermostat then it would end up costing as much as a new radiator. What would be the best option and do I really need the derale thermostat? Btw there is already a factory external cooler as well but it seems too small and I plan to tow occasionally.