I drilled a hole in the side of mine, but VERY close to the front edge. I don't think there were any lines in the walls to begin with, but I just wanted to be safe. In general, if there is a radiator on the back and your fridge cools with a fan blowing cold air in then there are no lines in the walls. However, if the back of the fridge is solid, with no radiator lines then there has to be a warm side heat exchange somewhere
. Those warm lines usually run in the walls, and radiate heat through the outer metal "skin" of the fridge.