Good luck with this, Pete.
It does seem that leaving a window open to combat condensation and air quality does rather negate the whole object of having an extra heater.
I was quite shocked when I saw the price asked for these. Maybe consider an accessory heater such as Propex (gas) or Eberspacher (diesel – but a slightly bit noisier than the Propex, I believe)? A bit more expensive, perhaps, but an infinitely better product.
Something to consider?
Good luck and as always, let us know how this idea progresses?
Barry