My 36" GE Monogram built-in refrigerator has the compressor on the top. It keeps building up ice on the evaporator coil on the bottom of the frig although we don't see any water in the drain pan. The ice build-up starts on the left hand side. I attempted to see if the drain was possibly plugged but it seemed free of any material. The wiring diagram shows a defrost switch but does not show a defrost heater. How does this defrost and what might be causing the ice build-up? Any advice would be appreciated.

Thanks