I posted about it on here last year, but the two year old DD of someone I knew was killed last year when she was trapped in her carseat and their car caught on fire and no one could get her out (took a long time to get something to break the windows and after they did they didn't have anything to cut the straps of her carseat and the flames were too hot).
I now keep a LifeHammer and car fire extinguisher within reach. The Lifehammer is on the back of my console attached with industrial strength velcro where I and DS1 in the 2nd row can reach it. My fire extinguisher is in my glove box which I can reach as well.
I also keep a full size first aid kit, jumper cables, 2 fleece blankets, foil blankets, glow sticks, waterproof matches, and a flashlight with extra batteries. I also have a diaper bag in there with extra sets of clothes and underwear for both kids, a wash cloth, microfiber towel, plastic bag, baby wipes, hand sanitizer, etc. I think I have some snack bars in there too. I'm in Los Angeles so no need for snow/winter gear (besides an ice scraper). I used to have some bottles of water in there but I don't think I do right now.