wally3b4

I am having trouble with very little air flow coming from the vents while running the heater. If I switch the temp control from warm to cool, the air flow increases dramatically. I can also hear a change in the blower motor rpm as if there is a restriction when the temp control is set to warm. I have some dash components disassembled and can see the blend door actuator fully stroke from one position to the other so at least everything up to the blend door is working properly. Can someone tell me how the air flows across the heater core and what I should be looking for next? Any tricks on how to access the blend door without tearing the whole dash apart?