How much do you want to mod your car? The 12v heated cups and such work reasonably well, but many of them are designed more to keep already warm things from cooling off too much.
However... For about $100 you can get a 12v DC to 120v AC inverter that is sufficiently powerful for you to simply take a small microwave with you and heat up whatever you like as easily as though you were at home. You'd have to decide between wiring the inverter in so the power sockets were inside the cab and dealing with getting out and opening the hood and hooking the thing up when you needed it. Which is more practical depends a lot on what kind of car you have and how much space is available in the engine compartment and passenger area.
If you do a lot of winter driving and are willing to invest a bit in a more powerful alternator for your car, you can even have the system able to run a full 15A circuit. Personally I find it to be quite nice to just use a heat gun to defrost my windows in comfort instead of scraping ice or having to wait for the engine to warm up.
If cost is your primary concern, my grandparents used to just stop and warm the baby's bottle on the exhaust manifold of the engine. Those are hot enough to cook on even, but the practicality depends on what engine the car has and whether there's a spot to set things. At one point it was even somewhat common to see modifications that attached a small oven box so you could cook things as you drove down the road, but cars had more space in the engine compartment in that era, so it's usually not a practical modification now.