Rear climate control is only blowing cold air out instead of heat?

Hello,

It sounds like you might have a faulty blend door actuator.
The blend door actuator controls the hot and cold air mix. A faulty actuator can get stuck in either the hot or cold position. Which will lead to only blowing out hot or cold air regardless of the setting on the dash.

The blend door actuator is located behind the center console.

https://www.2carpros.com/articles/replace-blend-door-motor

I have attached diagrams for your reference.

Please let me know if you have any questions.
Thank you.