Refrigerator. I have a GE Refrigerator (1999) Model No TBG18DAYJRWW.The fresh food compartment

Question

I have a GE Refrigerator (1999) Model No TBG18DAYJRWW.The fresh food compartment gets too warm. I then empty the freezer compartment, remove the back cover and the defrost coils are frozen solid. I manually defrost the unit and it runs perfectly for 3 weeks. Then I have to repeat the above.I have read the various articles and have narrowed the problem down to 1 of 3 components.The defrost thermostatThe defrost timerThe heater.When I look at the defrost coils there are 2 defrost thermostat units (L55-30) components. Why 2? and how do I determine which one is defective? Where in the refrigerator would I find the defrost timer?Thanks, Liam Devlin