It would be more heat out for the rear surely?
I also removed the rubber weather strip below the wipers underneath which opens it up a lot more too.
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There is an easy way to prove which way it is going. Do you have some wool?
Cut some short lengths of wool from 50-100mm and tape them onto your bonnet/windscreen. Tape them so they face the way the airflow might go. Remember to alternate which way they face and the lengths across your bonnet/ windscreen area.
My guess is air flow in. Like the ram air revears cowls on 70's American muscle cars
Ill try and find time tomorrow to find out gents!
Looks like it's time Marky paid up then !!