Keeping enough liquid water can be a challenge when the ambient temperature is always below freezing. Solar heating will keep the inside of your car well above freezing during the day, but you will need liquid water in the morning, after a night of outdoor temperatures. Melting ice in the morning will cost a lot of time, since water has a high specific heat, so it is best to keep enough water from freezing overnight.
A full and fully-melted gallon container will be partially liquid after a single night where the temperature stays above zero, but a smaller container, like a mug or your pack’s water bladder, will freeze solid. Try to thaw your larger water bottles as much as possible during the day, e.g. by placing them in the sun covered with a black plastic bag. If you want a smaller container of water, keep it inside your sleeping bag. Note that canned carbonated beverages can explode when they freeze — careful with your energy drinks! Keeping them in a cooler may help, but only if they start out reasonably warm.
Window condensation from your breath, always an issue, becomes a real problem in the winter, when it will form a thick layer of frost. This frost will then melt during the day and drip onto your clothes and upholstery, causing both short-term (frozen clothes) and long-term (mildew) problems. It’s better to crack your windows more and suffer with cold temperatures and breezes than crack them too little and deal with interior frost.
Drying wet clothes is a constant battle. Since you are wearing more clothes than during the summer, you need more space to spread them out, and since winter clothes are more expensive than t-shirts, you have to wear the few you own more often.
On long days you will have few warm, dry hours in which to dry clothes, so it’s important to make the most of your drying time. Immediately remove wet clothes when you get to the car. Drying your inner layers is most important, so place them in the sun or near a heater vent. Putting on damp or frozen outer layers is unpleasant, but better than sleeping in damp thermals. Take the insoles out of your boots, but don’t put the boots in front of the heater, as doing so may damage the material.
Since it’s difficult and time-consuming to eat food that requires bare hands or utensils, or food that can freeze, your trail meals may be limited to nutritionless, monotonous, concentrated energy sources like granola bars, pop-tarts, and trail mix. Focus on eating protein, fatty acids, and vegetables for dinner, likely your only meal with real variety.