I checked out a book called Winter Wise by Montague Alford. Here are some notes from the vehicle guide section of the book. I don't have a car but I thought I would share it with you and see what you think:

- Do not carry your sleeping bag in the trunk, where the vehicle is coldest and where you could not get at it if the trunk were damaged and jammed.

- Any non-running vehicle is a heat sink and offers no insulation from the cold. So you have to build a snow shelter like a snow trench or a snowhouse (adequately ventilated to prevent carbon monoxide poisoning... learn and practice how to build one) or have a tent...

- You need an Insulated pad under your sleeping bag to prevent heat loss through conduction. Not a self-infating pad but one or two closed-cell mats. A foam pad made of evazote. Vehicle seats and cushions offer good insulating pads for the inside of a snow shelter.

- In an emergency, black pepper is an effective radiator sealant.

Some items from the vehicle kit list:

- Axe and file
- Flashlight (headlamp!) equipped with alkaline batteries because they perform better at low temperatures.
- Tarpaulin (of sufficient size to cover engine hood and reach to ground on both sides)
- Spare fan belt
- Spare fuses
- Large orange garbage bags (you can fill one with snow and use as a door to a snow shelter)
- Thermos kept topped-up with a hot sweet drink
- First aid kit which should include a cushioned mask such as an Airway SealEasy, used for sanitary purposes when administering CPR
- Extension cord (electrical, heavy duty)
- Emergency heater (gasoline camp stove or the alternative already mentionned: roll of toilet paper soaked with kerosene or diesel inside a coffee can
- Ban ice (methyl-hydrate)
- Anti-freeze
- Window scraper
- At least two 4x4x12 inches wooden blocks.
- your PSK (the author carries a homemade candle stove with two nested cans and tea candles to melt snow and carries a swizzle-stick to stir snow and a plastic drinking tube to drink warm liquids from the candle stove without the need to remove the container from the stove base and also can serve as a fire coaxer (the tube lets you use a safer technique than placing your nose, and your parka ruff, within a few centimetres of glowing coals.)

etc. (I left out other more obvious items)

Good luck
François