We purchased a cheapo seat warmer for our car several years ago. It was a blanket kind of thing, that plugged in to the power thingy.
It was awful. The actual blanket-y thing wouldn't stay put and it was really a huge hassle. Plus - we didn't like the additional cords in the car.
Now...DH and I have the seat warmers in our current vehicles. Sucks for DD, 16. Her car is too old for seat warmers.
This is my most favorite option for the cars.
Unfortunately - my opinion of the after-market seat warmers isn't that great.