I have a black fleece blanket in my car. Great for warmth, but also padding cargo and covering things.

What are some Very Useful Things in your kit?

a decent flashlight or headlamp is often useful. I also like to keep a reflective, bright vest to wear over mu regular clothes if I am working on the car by the side of the road.

I rarely go anywhere without at least a small shovel on hand,maybe more depending on the situation.

You mean for the car? Man, a person could live pretty comfortably out of my car for a while! Especially now that it's winter I always have Wiggy's Pack Boots, a heavy Wiggy's bag, Wiggy's mittens, five other pairs of gloves including one heavy leather, one fleece and a few other assorted types. Also a small shovel, a few tools, a flashlight and a headlamp, a Resq tool (glass breaker), duct tape, a heavy splitting axe, two or three Silky Saws, a tarp and at least one tent. Yeah, a lot of stuff but part of that is my load out for camping so it just stays there.

Where do you put your groceries when leaving the store? Strapped to the roof?

I have a similar loadout in my car (minus the tent plus a standard FAK, spare fluid for the window washer, a couple of microfibre rags and an unhooking mat). I also pack one reflective vest in neon yellow for each passenger seat and two cheap blankets. In a station wagon that is barely visible. A weeks worth of grocery does not even take a third of the remaining space without losing passenger seats.

Hahaha! My car isn't huge but all that stuff actually fits just fine in my trunk with a bit of room to spare. Generally I put the groceries in the back seat, but meat and stuff I have enough room for in the trunk.

Interestingly though, strapping to the roof is a good idea, and a very popular one here in Montana. About every third car I see has a Thule "shell" on the roof. I asked a coworker what folks carry in those things and I'm told it's skies and snowboards mostly. Here in Bozeman I'm told there's very good skiing in the Bridge Bowl, the Crazies, etc.

Traded my Ram in a few years back for a Subaru CrossTrek. Love the thing but I had to reconfigure considerably as the interior space was a lot less for all the things I normally have in my vehicle. I still manage to have all I need on hand by using the spare tire well and the space under the seats etc. No room for a conventional blanket but I do have 2 top quality reflective survival blankets in a seat pocket. That conbined with a couple of sterno gel stoves should get me and a passenger through a night or 2 should I break down somewhere remote.

I’ve been eyeballing this cage thing on amazon I could mount to the roof rack. Would be ideal when transporting gasoline cans and other things I don’t want stinking up my car. Being a hatchback with fold down seats I got plenty of room but the last time I spilled a few drops of gas inside it took a week to air out the fumes.

I suppose it bears pointing out too that I rarely have anyone in my car except myself. Occasionally I'll give a coworker a ride but it's very uncommon that I have more than two three at the most. So if a few things are in the back seat it doesn't bother me at all.

The roof rack would indeed be handy, especially for things like fuel. I'm not too excited about the idea of a gas can inside my car! Not safe at all IMO.

If you have a tow hitch, you can also consider a cargo rack at the back.

This is what I got:
https://imgur.com/a/unnkOvO

I also got a roofbar and box, but i pretty much only use those for skies.

A heavy-duty jumpstarter. Quicker and easier to use than jumper cables and allows for self-rescue, and very handy indeed at CERT callouts for recharging laptops and smartphones.

A small inverter — I often road trip with coworkers who are working on their laptops while I drive, or sometimes I’ll want to recharge my laptop (but not use it!) while I drive.

The biggest and baddest snow brush/ice scraper I’ve ever seen available for sale. PSA: get the snow off every scrap of glass, off the hood, and off the roof. Blowing snow off your car into your own face or into the faces of folks behind you is no good.

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.