RE: Rear A/C Heat problem
After working on it today, I discovered a few things that pertain to my version of the problem. The servo that operates the flapper door is working perfectly fine, but the stop tab on the outer sheath that connects the servo to the flapper arm mechanism had snapped off at some point, which let the servo continue to spin, which then must have overpowered the flapper's arm and snapped that as well. As a result, the flapper was just sitting inside laying over top of the heater core.
I was able to pull the flapper door out of the unit to at least let some air hit the heater core, but since the air is not forced through the core, the air doesn't really even get warm.
I wasn't able to get the unit fully disassembled to assess whether or not it would be feasible to replace the sheath and the flapper door since they are both broken, but at least I know exactly what is wrong and what needs to be replaced (assuming I can find the parts and get the rest of the unit disassembled).