This could be a problem with your defrost actuator. Over time your vehicle's actuators, which are little plastic gear driven motors, through normal wear and tear will wear out. When an actuator finally fails, if it stuck in a position, that is where it will stay. Your defrost actuator, it sounds like got stuck in the open position when it failed.
I have included a link for you to go to down below:
It is a helpful guide for your vehicle's condition. I have also included vehicle specific factory repair and replace instructions in the diagrams down below. Please go through these and get back to us with what you find out.
Images (Click to enlarge)
Sunday, March 24th, 2019 AT 11:27 PM