Circuit/System Description
WARNING: This page is about a different car, the 2016 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 12 V to 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.