The real problem is that the insulation is becoming saturated with water and unable to keep the surrounding area from freezing. You can install this kit as a permanent fix.

Now, here's the kick in the pants: thanks to SZ, you can no longer buy the floor heater thru the link above-- it's left there because at the time of the original post you could and it shows you a photo of the kit. SZ decided they wanted to keep those parts profits in house so you have to buy it thru either SZ or one of their beeotches.

Source: Subzero Mod 550, defrost drain iced up