When Karin started doing on site calls, I stocked up her car. She's got a saw, axe, bolt, cutters, couple of shovels, blankets, a space blanket, jumper cables, tow lines, liferaft food and some basic tools plus duct tape and zip ties and probably loads of other stuff I can't remember. The problem is keeping water in the car though, since everything freezes at winter. Of course, in a pinch you can always eat snow. And I should probably check the flashlights in there, haven't used them in a while. One of the most important things though, in my opinion, is keeping a knife in the glove compartment. If you're stuck in the car upside down, you can cut the seat belt, and it can be used to dispatch wounded animals too. Or then you can get one of the seat belt cutters and keep it in the vanity mirror. Many people have them in their cars, but they keep them in the first aid kit in the trunk, what the heck are you going to do with it there? Sure, it's handy if you need to help someone, but you need to get out of the car first to do that.
*Edit* I keep forgetting about the 4 tonne cable puller I keep in there too. And then there's the first aid kit of course too.