Circuit/System Description
WARNING: This page is about a different car, the 2017 Cadillac ATS. However, it is still accessible from the selected car via links, so may be relevant.
When the HVAC control module receives a rear window defog request from the radio/HVAC controls, the HVAC control module will provide B+ voltage to the coil side of the rear defogger relay. This will energize the relay causing the contacts within the relay to close and provide B+ voltage to the rear window defogger grid, the driver outside review mirror and passenger outside rearview mirror heating elements.