Remove the headliner.
Disconnect the front drain tubes (A) and the rear drain tubes (B). Detach the tube clip (C).
With an assistant holding the frame (A), remove the bolts, then remove the frame.
Honda Odyssey Service Manual > Rear A/C System Description: Rear Heater-A/C Door Positions HOTCOOL(VENT)(HEAT/VENT)(HEAT) ...