Dennis, I'm glad that the pictures on my site put the case in a better perspective for you. It is hard to see in there and if it saves you or anyone time and labor, it's worth it.
Here are some additional pictures after I partially reassembled the box to try to help you out a bit more:
This is about all that the heater door will open on the bottom due to the fact that the arm hits the bracket on the case. Note the tube outlet to the right, above the bracket. This one and it's mate on the opposite side, go to the defroster and upper vents. I'll refer to it again a bit later.
This is a view looking down into the empty case. The core is at the bottom (shown in the next picture) and the motor and fan fill a lot of the space above it. The two side exit tubes provide air to the defroster and upper vents. If a critter got in there, he would have probably blocked of those as well, because there's not much room in there, but anything is possible.
Let's assume that he did and it's covering the core on top as shown here, then no air will get out of the bottom.
Lastly, this is looking at the opened door at the bottom where it would heat your feet. You could try to take the air hoses of of the sides of the case and blow compressed air back up through the bottom to see if it dislodges and debris on top of the core and out the side air tubes. maybe you could even snake a small vacuum hose in there to pull it out, but be careful! The cores are copper and not very resilient to being pushed and poked.