I'm 99% sure that your problem is the final stage blower resistor. The rear air vents have their own blower fan in the center console.

The blower resistor is under the passengers side of the dash, and can be changed in about 5-10 minutes. IF you use the search bar at the bottom of the page and type in blower resistor, there are numerous threads/posts about this, unfortunately it is a common problem. But the new one is an updated style and doesn't really fail. Just the original ones generally fail.

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