I just ran across this interesting article, when doing a google search for info on car's electrical efficiency. While much of this article is speculation about the future, there is still a lot of solid numbers about current car electrical systems (including the real cost of electrical power in a car).
NOTE: In the article above, the author assumes approximately 70 watts for secondary lighting (that comes on with the headlights) in a car. However, this is very variable with the car, with some cars (for example, my CRX) that use many "small lights" using significantly more watts than that 70 watt estimate. This is due to the fact that standard incandescent bulbs while never very energy efficient, are none-the-less even less energy efficient (in terms of light per power used) as you shrink the bulb size. So you will use more power to get the same light if/when you use many small incandescent bulbs, then if you use just one or two bigger bulbs. But you will be much better off then either option, if you use high efficiency LEDs instead.
BTW: On my Honda, the 1st light switch postilion turns on all the lights normally associated with headlights, except for the headlights themselves, including the secondary (now converted to LEDs) white lights pointing forward (which is different than a lot of American cars do it, as many American cars have the amber front turn-signals turned on solid in this switch position, which is NOT true of my Honda). Since I've got all those lights (except for the interior dash lights, which I still haven't gotten around to pulling things apart to replace with LEDs) converted over to LEDs, I now sometimes use that 1st light position as "running lights" in my car when I'm in a slightly marginal visibility situation. (i.e. I use this light switch position when I just want some lights on to make it easier to be seen by other motorists, but when it's still light enough that the headlights wouldn't increase my ability to see anything. In such situations, I think these "running lights", with low power LEDs in them, solve all the problems of being seen by others, without incurring the huge power penalty of the main headlights being on)...