People have all sort of electrical gizmo’s in the car – you plug your cell phone, use your nav aid or drink a cold one (soft drink!) straight from the on-board fridge. Some have TV screens and special lights put in. Not to mention the essentials the car came with – headlights, air conditioning, radio and such.
All those things ran on the car battery power, that in turn is charged by the alternator which is a small turbine that get’s the rotation from the engine. Now, given a certain speed, it is clear that if you activate another appliance you need to give the engine more power and fuel – and so you get lower millage. But how much? is it really noticeable? Well, as with everything it depends… lets talk in horse power since it is common in car talk – 1 HP is 745 Watts, now the battery gives ~12V and so if you run the battery output at about 60 Amps you use up 1HP. That’s about 1% of all your car power output and while it is true you need a lot of appliances to get that power, running a fridge, the air-con and some flat screens should get you pretty close.
About 2% of the car energy output goes to operate the car accessories, a big portion of it goes to the alternator. Keeping in mind that as electrical power plant goes, the car has a very bad efficiency in comparison to big power plants (http://answers.yahoo.com/question/index?qid=20080927100559AAq4lTM) and so it is better, economically, to charge your phone and cool your drinks at home then in the car.
Now, I’m not saying don’t use your air-con or keep your lights on low, just bare in mind that what ever you plug in your lighter socket or aux outlet(s!) – it will cost you at the pump, and if you have stuff that are connected 24/7 with no use – it is a good idea to unplug them, and even better to put them away when not needed.