A few years ago I realized that automobiles weren't magically invented with heaters already installed. By coming across random references to motor robes I became aware that car heaters were uncommon until at least 1940, and the earliest reported sighting of a car heater that I could find was in a 1951 Plymouth (thanks, Gary!)
I'm pleased to report that we can narrow the field down even further, thanks to "The Boys' Book of Engines, Motors, and Turbines" by Alfred Morgan. Published in 1946, the book lists "car heaters" among the devices in which a curious suburbanite boy might find an electric motor...and since their inclusion in the list is totally blase, I assume that they were quite common by that time.
Therefore we can safely say that car heaters became standard devices between 1941 and 1945, based entirely on anecdotal evidence and my sort-of-quirky and extremely lucky reading habits. Anybody care to find a patent or a catalogue to back me up?
PS: This book is fabulous. When I started reading it I had no idea of how engines, motors, or turbines worked, and now that I'm halfway through I even know what a camshaft is, how hydroelectric power is harnessed, and that if you try to blow out the fire in your miniature steam engine you'll scatter burning alcohol around the room and "singe your whiskers."