I had an old Dodge Aries K-car with a broken heating system. It would have cost more than the car was worth to fix. So for $30ish I found an electric throw blanket with an adapter to plug it into the cigarette lighter. It kept me toasty warm in stop-and-go traffic where even the best car heaters would have trouble keeping up.
And in searching for that I found this, which looks even better. It costs less, warms the whole car, and on icy mornings you could set it on the dashboard to help you get going sooner.