While I built a battery warmer for the Electro-Metro a while back, I never got it installed.
There was some sort of a problem that was setting off the GFI on my charging power outlet AND the GFI in the car.
I took the cover off the warmer, fixed the problem, and installed the warmer in the car.
Running the warmer for an hour or so seemed to bring the batteries up to "Not Cold" temperatures, which is all I am trying to do - just get the batteries somewhere between 50 and 70 degrees F.
I will still need to experiment to find out what the ideal run-times for the battery warmer are at various temperatures.
Since the battery box is INSIDE the car, any heat escaping from the battery box should help to heat the cabin (at least a tiny bit!)