Yeah that is for the block heater. When it gets cold you plug it in at night to keep the engine warm. Some have battery blankets too since a frozen battery wont take / keep a charge.
I got tired of digging for mine (Mayfair just don't) so I mounted a plug: http://www.genosgarage.com/prodinfo.asp?number=WD-BHP
I run a timer on the outlet so it doesn't start warming it up tell about 1am (I leave around 6 so it is plenty of time).
Best part is the windshield never ices up that way
There is nothing worse than trying to get up on the hood of this beast in 2 inch heels to scrap ice (again Mayfair just dont).