I have a problem with the cabin airflow. The only way I can get any airflow from the heater fan into the cabin is with the recirc air switch in the on position,turn this off and there is little too no airflow with the fan on any speed?
Model is a 2002 1.8 TDI Zetec

On selecting the recirc air switch you can hear the flap opening and closing ok so it just looks like I have a blockage somewhere in the external air vent. Can anyone give me any advice on where to start looking?

Thanks for any help

Owen