Seems to me there is a lack of understanding how fans cool things on here. A fan just moves air around, what causes cooling is the evaporation of water that the moving air causes. By putting a fan in a brew fridge you are just moving warm air around in the enclosed space in the fridge, unless there is some water in there to evaporate and thus cause a cooling effect. A pc fan works slightly differently, the pc produces a lot of heat so the fan moves air that is already cooler over the hot components. The heat transfers from the components to the cooler air and disperses it, a bit like a wort chiller transferring heat.
I always just assumed that when people make a brew fridge, the fridge is plugged into the cooling socket and comes on when cooling is required. The area in a fridge is so small anyway it is very unlikely there will be any benefit to move the air around with a fan.
I am planning on doing a brew fridge next year and have a heat belt. I was planning to have my STC100 external to the fridge with teh frisge plugged in for cooling and heat belt for heating. The temperature sensor and heat belt will be put through a cut out in the door seal which I will then reseal with blutac (low budget solution). I am aware there may be a plug in the back that can be removed but not sure its on all fridges and dont know exactly what to look for so looking at the easy route. As long as its properly sealed. Is there any reason this wont work?