No air out of rear vents, AC fine in front vents

Front vents: AC cool, air volume good.
Rear vents: No air volume at all (specifically all 4 vents in the roof)

What I've already checked:
- Dashboard settings (Rear AC is on)
- Checked Rear Blower Motor fuse (ok)
- I hear the Rear Blower Motor running (sounds like it is running at least)
- Took off rear panel to superficially check motor (did not disassemble
motor).  Motor appears to run, ducts at blower motor are connected.

Things I'm thinking about:
- Duct Damper might be stuck, however I'd think that even with that damper
closed, the blower motor in the rear would simply push some sort of air
through those rear vents.  That assumption might be wrong, hence why I'm
here as I don't know what the next thing to check is.

Any help would be greatly appreciated.