I'm in Cleveland, it's not the Icebox of the Nation, but it does get cold enough to freeze beer. I'm going into my first winter with a kegerator in the garage and was faced with the same issue.
My solution was simpler, but probably a bit more wateful of energy. I took advantage of the light that is already in there. I pulled out the switch that turns on the 40 watt bulb in the back. I was able to pop the switch out. Then I trimmed some plastic to give me a little slack wire so I could get it into the fridge and out of the way of the door. Finally, I duct taped it to the ceiling of the fridge. One of the adavantages of having a kegerator that's a rusty POS somebody on Craig's List was giving away is you don't mind cutting it up a little bit or having eelectrical wiring duct taped to the ceiling.
I think this is going to prove to be a very simple and effective solution. 40 watts seems like it should be plenty to keep a very well insulated box above 29 degrees or so. I'd guess it's actually more than I need.