My GE Heater element AP3183311 failed and I replaced it. The new one appears to have overheated and has melted the plastic ends that hold it in place along with the plastic trim on the cover panel. Is This a Defrost Thermostat problem or a main board problem? How can I tell if it's the thermostat, the board or something else. Thank You,:)

You got to remember the defrost thermostat on these models is a 140 degree thermostat so if it trips there is something else wrong such as a bad evaporator thermisotor or main board.

What I would do if I suspected the heater stayed on too long is check the evaporator thermistor and if it is ok replace the main board.

However there is a service bulletin about the defrost cycle lasting too long that suggested to replace the main board and the thermisor. They also suggest removing the piece of foam gasket behind the evaporator cover at the top (right at the fan housing. (This foam isn’t on all models). They say that the foam will retain moisture freeze, then the thermisor will read temperature off the foam and keep the heater on too long.

GE Refrigerator Repair Guide