Frig heater element not turning on to defrost evaporator

GE GSS25SGMBBS Refrigerator is not defrosting evaporator icing up. I replaced all 3 thermistors , the temp thermostat sensor and heater element. I tested the element by bypassing the switch on circuit board and the element heated up. However it doesn't go on by itself . This model frig the temp control is built into circuit board . Is there a way to test to make sure built in temp control is working and trying to turn on the heat element when needed. I know the element suppose to come on a few times a day , but it's not because after replacing all these parts it's still icing up . Thought there would be way to test the temp relay in circuit board . I visually inspected the board and all looks good unless I'm not seeing something . Is there a way of testing this relay and where is it located on the board?