I have never had a vehicle that had a plug-in for a heater in the engine. My question is when should it be plugged in? It's a 2005 PSD with 60k on it and I only drive it about once a month during the winter. It rarely gets below 27-32 degrees in the coldest times here in Central Texas. Should I leave it plugged in all the time when not in use?.....only at night when it gets below X degrees? Any help or advice is appreciated. Thanks.