Hello,I will be glad to try and assist you with this,Welcome to Just Answer.
In your under hood fuse box should be a fuse labeled BATT 3 which is a 40 amp fuse.If this fuse is blown it will cause this issue here,check and see if it is blown and replace it if it is.
There is also a relay for this rear defrost,this relay is located in the under hood fuse box as well,it is labeled REAR DEFOG.With the engine
running have someone switch on the rear defrost and feel to see if you can feel this relay click on,if it doesn't try tapping on it with the handle of a screw driver and see if the rear defrost works then,if it does then this relay needs replaced.
The fuse and the relay are both common causes for this fault here.
Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!