The demisting/defrosting device must be either an electrically heated windscreen or a fan assisted warm air system using ducting and vents to direct the air onto the windscreen.
Both systems will be judged on their ability to adequately clear the windscreen to give the driver an adequate view of the road ahead.
Interestingly the rules state that if a ‘warm’ air system is being used then it has to be fan assisted. What it doesn’t say is that a cold air non fan assisted system is acceptable but common sense would dictate that this type of setup wouldn’t adequately clear the windscreen.
Any warm air fan assisted device must be integral to the car, ie a hair dryer pointing at the windscreen would not be acceptable! 