Alan, there are two types of Chevy factory accessory heaters for the '54 truck; one is a "Fresh Air Heater" which attaches to the kick panel and circulates fresh air through through the intake louvers on the side of the passenger cowl and then blows that air over the heater core and into the cab, the other is a recirculating heater which is attached to the firewall and simply takes cab air and "recirulates" it over a heater core.
There is a peculiarity to the '53/'54 truck fresh air heaters in that they added a recirculation feature which incorporated a double door flap that would close off the cowl intake and open up a cab air intake to recirculate cab air too.
I'm a GMC man and will further confuse this issue by stating that GMC offered three heaters for '54 GMC trucks: a Special Heater (recirculating only), an Airflow Heater and Defroster (fresh air heater), and, an Economy Heater which was recirculating and the least expensive way to get some heat on cold feet.
The link below illustrates the GMC heater options for 1954:
1954 GMC Heaters
