Interesting. I would thought anything out from center front vents would be the same going to the center rear.

Have you tried closing all the front vents and see whether heat goes out from the rear vents? I.e. manually set air flow to center position on the idrive and closed all the manual flap via the thumb adjuster?
And check whether rear vents blow the heat.

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Here's an odd thought....Seeing as how these vents range from 50-100 bucks on ebay, have you tried to swap the vent out?

I was thinking I would try that. I have a feeling the hot/cold slider may be broken on the rear vent.

I think what you're experiencing is common and "normal". We don't get heat through the rear (not 4-zone climate either) unless it gets real hot in the fronts first and it somehow circulates to the rear through those ducts eventually. That cold/hot wheel toggle in the rear has served no purpose since I can remember.