I just picked up my 95 YJ SE last week and just noticed that it has a plug inside of the grill. The owners manual says that it an optional "Engine Block Heater". My question is , how is engine block heater susposed to be used? Do I plug it in overnight on cold days? Do I plug it up an hour before I start the engine? What does heating the enging block do? I apologize in advance for asking what most of you consider "rookie" questions, but I am a rookie when it comes to Jeeps as it my first.