Being a "careful" Yorkshireman I run a timer controlled extension lead connected to a 2kw heater into
https://www.amazon.co.uk/Lloytron-F2...qid=1536955510 the car, 15 minutes clears any frost off all the windows and heats the interior.
I've used it for the last 2 winters to great effect, an extra 10 minutes even clears snow.