Kenmore Top Mount Refrigerator - leaking water...

Question

Our 6 year old Kenmore Top Mount Refrigerator is leaking water from the Cold Control Defrost Timer/Automatic Defrost Control Housing. I have had repairmen out twice before on this issue. Both times there was a solid layer of ice in the defrost drain pan. Both times they poured hot water down the drain line (?) to clear it out - apparently something had gotten in and clogged it?? The last repairman left a piece of aluminum wire in the drain hole - to prevent future clogs. This time around I shut the refrigerator off and defrosted it. I poured water down the drain line and it seems to be clear now. Anyway, why does the drain hole keep freezing up?? The last occurence was just about a year ago.