In the 403 the heater is not much use unless a window is open. It is hard
to believe that the cabin is that airtight, but letting air out
considerably improves the flow coming in. This is particularly important
when the demister is needed. Perhaps, like Ashley, I have been too
effective in closing gaps in an effort to reduce noise. It is also
difficult to close a door if all windows are closed. The rear side windows
provide the best escape route for air, as this allows the warm air to pass
through the cabin. Opening these is a struggle from the driving seat, and
that is why I rarely open them.
Has anyone devised a "throughflow" system like modern cars have, which would
not involve visible bodywork changes?

Mike Davies.