I think you guys are talking about 2 different things. The fan that queball is talking about moves air across the refrigerator condensor improving the heat exchange to the outside and improving the cooling ability. The fan that fbman is talking about circulates the air inside the fridge so you don't get cold or warm spots. I've heard these are available pretty cheap and do a good job. A friend has one and claims it works great but I've not tried one myself so I can't for sure. Camping World has a couple of them for less than $20 but from the reviews they are kinda cheap quality and break easily.
2011 Chevy Silverado Crew Cab 4X4
2009 210 WBS