When we were driving the F abroad, it quickly became clear that without using the fan there is not a lot of flow through the vents. Even at speed, the natural flow is small.
Looking at the external air box it's easy to see why.
The only access for air is hidden away in here, with a metal cap over it (to stop rain entering the heater.)
An easy way, I thought, to increase airflow would be to open another access hole and direct air from the front of the car into it via a flexible pipe.
The box is made of thin plastic and using one of these,
it is easy to open a hole.