With my late-model heater/air plenum sitting on the conference table in my
office, I noticed that the bottom flaps are not only removable by removing
the four plastic plugs that support the shaft as it sits through the
housing, but that it would be relatively easy to remove same with the heater
in the car. I can't prove it until I put the heater in the car, but it would
go like this:
First, pull off the lower heater cover. Then get a flashlight and find the
four clips that pull out. I will send anyone a close-up on request. Its
difficult to see them, they are black plastic like the edges of the heater
plenum they are contoured exactly like.

Make a note of how it comes out, if you put it in backwards it won't seal.
Beware that the metal paddles that form the vent closers are not perfectly
flat, there is an offset on each side that will force you to guide the
blades right between the raised plastic channels in the case. A mirror and a
screwdriver and lying on your back might help. Anyway, a lot easier than
pulling the dash.

You cannot get to the upper vent flaps this way, only the ones that control
hot air spilling into the cabin when they are supposed to be closed.

Jim