(Shaker Heights, Ohio)
Dad was raised on a ranch in Wyoming. When he married Mom, & she - the city girl from Back East - asked what the seasons were like, his response was "Two. 4th of July & Winter".
Because of him, my sister & I were raised, despite now living "Back East" in urban settings, to have our Winter Survival Kit in the car comes the 1st of November.
Dad was adamant that we use either wool or - nowadays - polar fleece or the like. Not cotton or down - neither will keep you warm if they get wet. And they WILL get wet. Extra socks - wool. Extra gloves, mittens, hat & scarf. Some sort of candle for both light & heat - preferably not the "smell pretty" kind.
Matches in a waterproof container. A bottle of water. Dried fruit, nuts, packaged candy. A friend suggested jerky - a fabulous idea, as it all comes pre-packaged, now. TP, of course; we girls can't do without that.
I suggested to her, as she drives far more than I - to keep a crossword puzzle book and a mechanical pencil. A couple of plastic bags to put on your feet. Keep an old pair of flat shoes if you wear heels regularly. Slacks - wool, if you can - & I keep a turtleneck sweater as well.
Yeah, a lot of stuff. But, I DON'T keep it in the back of my truck. I keep it all, in a couple of those giant zip-lock bags, behind the front seats of my truck. As Dad pointed out - the last thing you want to do if you run off the road, get stuck in a blizzard, get rear-ended, run out of gas (& we never travel with less than a half tank of gas), get stuck behind a pile-up on the freeway, or anything else, is to have to GET OUT OF YOUR VEHICLE to try to access your survival kit.