as the Icy weather is here a simple thing I used to do before the Truck I used to drive did not have an overnight heater, I used to stick a newspaper on the inside of the Windscreen, tuck some papers on the side door as the door closes, with a car just raise your wiper arms off the screen, this may help you by not blowing a fuse with you turning them on and the wipers sticking fast to the glass.
If the Fan Heater is connected to your mains OK, if it's the type that's run off your battery, get your engine running first otherwise it MAY flatten your battery.
another point if I may, if the car has been running through rain & it's going to be parked up for a few days, leave in First / Reverse otherwise the handbrake Pads / Linings COULD Freeze up, put a choc at the front / back of the wheels.