No there is no ice build up.
Sorry I misread the last question there is ice on the back wall none of the other walls but a lot on the back wall. I'm not sure about the temp there is no gauge.
With 96% accuracy the problem is going to be the heater went out for the defrost system, Having a multimeter to make measurments would confirm this but these units a prone to heater failure. This is installed behind where the ice is built-up on the back wall. this is part number wr51x10101, and you can go to the following website to order this heater.
BONUSES ARE WELCOME but are not mandatoryChas