[cwillu]

Off the top of my head, what you want in a warm car when the doors have opened and a shivering passenger gets in is very different from what you want when the car is cold and you're sitting in a cold seat is very different from what you want when a burst of mild humidity from a conversation or bag of hot food hits the windshield is very different than what you want when you want the interior to warm up as fast as possible without regard to windchill because nobody is in the car right now.

The issues are similar to tying the seat warmers into that single control.