I think that will be MFI
The back wall (#11) has to come off to see the start of the drain pan under the evaporator/cooling coils...
#20 is the pan under the evaporator/cooling coils...
...remove any ice and flush hot water down the drain system.
A turkey baster can be used to help clean out the drain system. Fill the baster with hot water and "blast" it into the drain hose to help flush it out. A piece of wire or pipe cleaner can also be pushed into the drain hoses to help clean then out.