Defrost thermostat connector. cannot disconnect the white connector from the defrost thermostat to

Question

cannot disconnect the white connector from the defrost thermostat to the defrost heater need help

I have had this ref. for less than ten years and have had service out about every 24-36 months. Service tells me not to block the air flow vents and I do not. I have replaced the control panel 3x under service warranty contracts. I think I am having defrost problems and the lack of air flow over the evaporator coil. About three weeks ago, I manually defrosted the ref. and replaced the upper fan and all was good again. For the first week the ref. actually turned itself off for a few hours and drained a little into the evaporator tray. The last two weeks, the upper fan has kept running all day and all night. The lower fan has cycled on and off.
Its been three weeks and I am having problems again. I noticed the ice starting to sweat and the over temp. alarm came on this afternoon. Turned the ref. off and removed the evaporator cover and noticed the bottom 1/3 of the evaporator coil was frozen over heavy frost. I think the defrost system is not doing its job.

Question - What controls the defrost system and how do I test the system? What else could it be. PLEASE HELP!!!!