If everything goes as planned, you'll see something like this. This is the plentum chamber itself, where the evaporator core, heater core, and temperature blend door are. Let's crack that case open and see what's inside...15 - 8mm bolts...
Pictured here are the evaporator and heater cores, and the blend door. The blend door is a snap fit into the case. Pull back on the frame of the door on the passenger's side of it and lift the whole shootin' match out. What's wrong with that door, you say?
This one actually wasn't split all that bad. The blend door motor went to the parts house in the sky, and while peering through to see if the blend door motor was running during diagnosis, I saw this split in the door. If you've got EATC, this will be accompanied from time to time with DTC B1249 in the self test or test run with a compatible scan tool. You may also hear a popping noise from behind the dash.