The fresh air heater (made by Harrison) was an option and attaches to the right side cowl and takes in outside air through the louvers. It has a motor protruding through the firewall and has a defrost control rod attached to the underside of your dash. There is also a heat control valve that is connected to two heater hoses under the hood. The blower motor is controlled by a switch that is attached near the defroster/heat control rod.

The other heaters are stand alone types, sometimes round or square and hang underneath the dash. They can also be fitted with a defroster motor and vent. (I think that this is the type that you have by your description)