Amazon accepts returns for basically any reason and makes it very painless to get products back to them, that should not be a problem. But I'm not sure batteries will cut it here anyhow. I have battery heated garments, they were $300 each for the pants and jacket, they last maybe 3-5 hours. I have tried to spend less in the past but this is about where the bar is if you want something durable that will last a long time without breaking. You'd need both, as well as two spare batteries ($80 apiece) to keep warm all night. That's simply where the limit of li-ion energy density currently is.

It seems to me that you have two problems. The first is how to economically generate heat. The second is how to trap and conserve it. Running a 12v heated blanket off your cigarette lighter socket would work but you'd risk running down your starter battery too low unless the engine is running in which case you may as well just use the cabin heater and keep a jerry can handy. But that could get expensive quickly as well as arousing suspicion.

However, you may have noticed many places have unprotected outdoor power outlets for powering cleaning and landscaping tools, or at the bases of trees in cities to power christmas lights. Campsites have these of course, as do many parking lot street lamps at hospitals for charging electric wheelchairs. There also still exist some free legacy EV chargers with standard wall outlets for electric motorcycles.

If you can use a hole saw, drill, nuts, bolts and silicone sealant to fit a weather proof male passthru to the outside of your car, you can plug it into one of these public sockets via an extension cord. Like hooking a boat up to shore power, or an RV to campsite power.

Then you have power inside the car you can run a full size electric blanket (or even space heater, though it's unsafe to while sleeping) off of, all night long. If you buy a PHEV badge for your car, you can tell anybody who asks why it's plugged in that it's a plug-in hybrid conversion. Important to note, you can't stay parked in an EV charging space for too long, there are time limits and fines in many states. Good place to charge up devices for an hour or so, however.

Now, how do you trap the heat as long as possible, once you've generated it? Mylar is magical stuff. It's the most efficient insulator I know of. It's used in space blankets and bivvy sacs. These are cheap, adding them as a layer to your existing sleep setup will greatly increase how much heat gets trapped. Mylar was originally invented to be a lightweight insulator material for the Apollo lunar lander (hence the crinkly looking gold foil stuff around the bottom stage)