HVAC Control Module
WARNING: This page is about a different car, the 2020 Ford Transit Connect. However, it is still accessible from the selected car via links, so may be relevant.
The HVAC control module uses a PTC request signal that is carried through a hard wired circuit from the RHVAC control module to the front HVAC control module to request heat from the rear electric booster heater. The HVAC control module sends the PTC, PWM output request through a hardwired circuit to the rear electric booster heater.