I am getting ready to pull the back panel out of my freezer to check the defrost heater and thermostat. Right now the refrigerator isn't cooling properly and I have a frost line building up the rear wall of my freezer. However, have read that sometimes the problem is with the motherboard and was hoping to get detailed instructions on how to check that as well. Should I start by checking the heater/thermostat or by checking the motherboard? Also a relatively stupid question...how do I get the rear wall of my freezer out? Thanks for the help!