Hi, Only 3 parts control the defrost cycle, the defrost thermostat, defrost heater, and control board. If the defrost heater ohm tests good and the defrost thermostat ohm tests good while its frosty(or in a glass of ice water, waiting at least 5 minutes before ohm testing it) then the problem is your control board.
I know you said you already replaced the defrost thermostat, but I'd still ohm the new defrost thermostat for continuity(0 ohms) WHILE ITS FROSTY(or in a glass of ice water, waiting at least 5 minutes before ohm testing it) while the refrigerator is unplugged.
Here's the defrost thermostat for your model:
5303918214 Defrost Thermostat
There is a video when you click the part link to the defrost thermostat, it shows where its at.
Watch this video:
Defrost Thermostat Ohm Testing Video If it's frosty you don't need to remove it and put it in ice water to ohm test it.
If it has no quick disconnect, you will have to cut the wires and splice back the insulation a little, don't cut to close to the defrost thermostat itself as you will need 2 wire nuts to connect the spliced wires back together.
If both the defrost thermostat and defrost heater ohm test good, then it can ONLY be a bad control board.
Here's the control board for your model:
A01078804 Control,electonic,w/cover
Jake