A packing blanket from Harbor Freight, the small version, that cost about $4 on sale. It covers stuff I don't want others to see and steal, pads things when transporting rough objects, covers the seat when my dog has muddy paws, is used as a pillow, or blanket on longer trips for passengers (I wash it to get the dog prints off first), it can be used as a stretcher or given away to someone homeless or cold as it is so cheap.

I have a large mag light next to the seat for self defense.

For accidents, besides bringing a small first aid kit, there are heavy vinyl coated gloves, vice grips and knife for extraction.

For breakdowns, tools, wool blanket, cord and tarp, flares, tow strap, jumper cables.

To meet US Forest Service requirements for traveling in the forest, a shovel, axe and collapsible bucket. Also a small bow saw with both wood and hacksaw blades (for getting past blowdowns and out from behind locked gates).

Winter I add tire chains, a brass window scraper, set of waterproof clothes, blaze orange hat and waterproof mittens, heavy wool shirt. Sometimes a snow shovel as well as the metal one that lives there year long. A full set of studded snows.

My summer car kit also includes work gloves, a small flashlight, umbrella, filled water bottle, backup gps and snacks.

...I have lived cold places where a zero degree rated sleeping bag was not sufficient.

That's cold indeed! Most of the places I have lived (and my current home of Bozeman, MT) also get down much colder than that. Still, a zero rated bag that's honestly rated would probably save your bacon if you were stranded in a car. First off you could put on any and all clothing you had with you, which presumably would include a winter coat, cap and gloves. Second, the sets of your car would insulate you from heat loss by conduction better than any ground pad would. Lastly, even without power or heat, the interior of your car should trap some heat and at least will keep you out of the wind.

Every degree you can gain or maintain can be critical.

A great reason to include handwarmers, blankets, a hat, etc.

One of the few time I might use a candle (with sufficient ventilation) instead of a glow flashlight is in a car -- because it creates heat.