On a 53-55 Advance-Design truck outside-air "De Luxe" heater (AirFlow, Fresh-Air), there is no defroster cable (as was used on the earlier style, similar "De Luxe" heater).
The cable in 53-55 is used to open and close an interior-air/exterior-air door on the right-front face of the heater (to regulate between blowing/heating all outside-air to all inside-air).
The 53-55 defroster-lever is hand-controlled (or, maybe, foot-controlled) using the lever on the left front of the heater unit (as used on the recirculating heater).
