I've converted an old freezer to a wine cellar, using an independent thermostat, and have been very pleased with the results until just recently, when the cold weather here in the NW set in. Having located the cellar in an unheated area of my house, the low outside temps (high 30's) have been sufficient to drive the internal temprature of the cellar down to around 50 degrees or so. As the independent thermostat is set for 55, and is meant to keep the cellar from becoming too warm, it's no longer a factor. While I believe 50 degrees is too cold for my predominately "red" cellar, I'm even more concerned that, as our winter progresses, the cellar temps will go even lower. While the freezer is adequately insulated, it has no internal heat source, and so I'd expect internal temps to gradually follow external ones, and that won't work.
My first thought was to install some sort of low consumption heating element, perhaps even a 60 or 100 watt lightbulb left burning continuously, but then I thought I'd ask for advice.