Amana Model ABB1924BRW fridge Ice-build up on both rear wall and drain

[Winston8]

I have an Amana fridge Model ABB1924BRW that cools fine in both fridge and freezer, but it has ice in bottom covering drain and big ice ball at upper right hand corner enclosing defrost thermostat, copper tubing including where capillary enters bigger tube. I thawed out big block of ice and drain, after 1-2 days drain is clogged with ice again, I have not open the panel to check the upper right corner yet. Any suggestions would be greatly appreciated. 

[Winston8]

I should add I run the service mode and found no errors, evaporator fan works fine; when forcing defrost, compressor stopped, I could not tell if defrost heater works well or not.

In the last thawing process, the drain hole is definitely open as .a puddle of water in the drain pan.