Dave Liquorice pretended :
That would frequently just cover the car in ice and/or create an ice
rink around the car... Aerosol deicer on the side windows struggles
or even freezes at times. Start the car, press the auto defrost
button. Wander round clearing the side windows, lights etc. By the
time those have been done the screen is either clear or the wipers
can just push it off.

I used to always do the warm water trick, never a problem.

Ford had the patent on the built into the screen heater wires, which
prevented other manufacturers using the system. I think Volvo were
allowed use of it. They worked great, when I owned Ford Granadas.