One thing I always ask anyone on this subject to make sure of, Is that they can access their kit without getting out of the car!
Most of us do this without even thinking about it, but there are the odd few who drive saloons, and suddenly realise they cant get to their sleeping bags, once they got stuck in a snow drift.
The most vital thing I would carry would be my mobile phone (plus its car charger in the glove box). With the phone I have GPS to send my exact location, or to walk to safety), a phone to report my breakdown, a spare radio, all the outdoor guides I could want, a spare light source (plus I have torches that sit in the cigarette lighter and charge while driving, as well as a head torch in the glove box (as hand held ones are not much help when using your hands to dig yourself out/working on the car/giving first aid/cooking etc lol), a means of hearing weather forecasts or news.
I would also have all the usual stuff like a sleeping bag, shovel, food and water. One thing to make sure of with your water is to remove some of the water if its pre bottled water, so that if it freezes it wont burst the bottle.
If you have boil in the bag food, and your cars engine will still run, then you can always heat them directly on the engine. You must be competent enough to do this, and no where all the pulley belts are to avoid injury. Failing that you can heat them to some extent by placing them over one of the heater vents.................Just remember to have packed your favourite spoon too lol
If any of you are interested in using cigarette lighter socket torches, it is worth noting that many cars can have their cigarette lighter fuses set in two positions: one for always live, and one for ignition only live (so the device will not drain your battery when the car is turned off)