Ok, while a good start, those of us living in real bad situations need more.
I've been living in the middle of nowhere, up here in upstate NY south of the lake for my whole life. I've had my share of winter experiences, accidents, hiking, etc. Even including walking miles to a working phone. Lake effect here is no joke.
First, I can tell you that the trunk of a car isn't the best place for a lot of things. If you get stranded, or in a spot where you can't get to that kit in the trunk, and you can't fold down your rear seats, you're sunk.
Now, additional items to think about or change.
Chemically activated warmers. Those little heat packets in the hunting/sporting sections that will heat up if activated. Cheap, reliable, quick heat if you need it.
Jump pack. No excuse why you shouldn't have one. They make ones that have AC converters, air pumps, USB outlets, all in one.
Flares. Either electronic or traditional. If you are stuck in a ditch, and can't get out, the last thing you need is someone mis-judging the same corner you did, and running you over.
Shovel. They sell small plastic ones that extend out. Good if you're stuck but can possibly get your way out.
First aid kit. Reasons should be obvious.
Battery powered strobe, or some other emergency light that blinks. Will catch attention of passers by if you're stuck, warns other drivers to steer clear.
Tow strap. Even if you're not stuck, you might help someone who is.
Jug of ice melter. Good for getting off ice, or giving you traction in some cases.
Fire extinguisher. (Especially if you're carrying camping gear/propane/etc.)
Washer Fluid. (You always run out when stuck behind a big truck spattering you for miles on end it seems)
Extra quarters. (For pay phones, air pumps)
Notebook and pen. (If you have to leave your car, write down contact info.)
Spare key. (On you, obviously. Last thing you need is to lock yourself out of your car in sub-freezing weather waiting on AAA or a locksmith).
A pill bottle containing 2 doses of any meds you are on. I actually have a more flat shaped "pack" I carry with me that has my Rx meds, as well as a few OTC ones. (Do not leave in your car. Carry it in your jacket, pants, etc.)
Caffeine pills. Shorter days mess with your sleep cycle, and sometimes you might feel tired and need a jolt, but aren't near coffee.
Warm hat. You lose a huge amount of heat through your head. Preferably one that covers your ears.
Obviously, a cell phone. But also remember a charger.
AAA card. In my case, on our gold family plan, its more than paid for itself in just one tow.
Additional prep ideas....
Make sure you keep your door seals coated with a silicone based spray like armorall or even better black magic. Keeps any moisture from sticking seals together and icing your doors shut. Also remember to spray your tires. The cold, plus salty roads dries out the rubber on the sidewalls.
Spray locks and hinges with graphite spray to keep everything from getting stuck.
Make sure you can turn your lugnuts with your wrench. Last thing you need is a flat, and you can't even budge your lug nuts to put on the spare.
Bring an assortment of music. Even if you normally listen to high tempo rock, have something that calms you. The last thing you need while driving in bad slippery conditions is fast tempo music that amps you up and makes you anxious. You want to be aware of conditions, and be able to react quickly, but be calm and drive in a laid back state.
Last, I think.. if weather is gonna be really bad and you have to go somewhere, let someone know where you're going, and when you expect to be back. Make a travel plan and stick to it.
Above all, use common sense. Drive slower, counter-steer into skids, don't brake in the middle of a turn, make your moves smoothly instead of quick corrections. And watch the weather. Just because it looks fine out your front window, doesn't mean its not a blizzard where you're headed. The best tool to have is up in your head. If you drive smart, most likely, you wont need anything else to save your stranded ***.