Kenmore 73503 water leaking into freezer

We have a Kenmore 73503 double door refrigerator with bottom freezer compartment. Water will intermittently drain into the freezer compartment and freeze on the bottom. I understand that this should be draining into a pan that is located in the bottom rear of the refrigerator and upon inspection, this pan is completely dry. Using a stiff piece of wire, I probed up into the drain tube checking for an obstruction and it appeared that there was ice blocking the drain tube. I tried without sucess to clear the ice with a hair dryer. Finally, I unplugged the unit and allowed it to defrost completely (it needed a good cleaning anyway). Upon powering the unit up again, it worked for a couple of days before I observed water draining again into the freezer compartment.

I'd like to get at the drain tube further up inside the refrigerator but the inner sheet metal (behind the removable cover) appears to be sealed. I cannot seem to find any sort of service manual online either.

Why would ice be forming in and blocking the drain line? Does anyone know how to get inside the unit to where I can inspect the beginning of the drain?

Thanks!