Update: Local ATV shop owner has a Defender that he plows with. His take on the heater issue is to get full doors first, use the machine and see if a heater is really necessary for what you are doing. When the rad fan kicks on, the cab will warm up due to the fan and openness of the front chassis. Unless it is brutally cold and the windshield won't stay clear, a heater may not be as important as we think. I appreciate his input, as he does a lot of customizing and upgrades and should know what is the best use of resources (MONEY!).