That blue heater in your picture has a neat feature which the earlier ones did not have. The cable you speak of, converts this heater from a fresh air heater to a recirculating heater, or a combination of the two depending on how hard you pull the cable.
The tall narrow door which you can see is actually the air intake for the recirculating feature. When this door is all of the way open, a linkage closes off the outside air from entering from the side cowl vent.
