I'm in the process of finishing up a Nostalgia KRS-2150. Is yours the same model? I was going to do a fan w/hose up into the tower for cooling but decided against it. The "twist-lock" base on the Nostalgia kegerators was my reason for not doing it. Even though the tower is 2.5" the base only leaves a 1-1/4" - 1-1/2" hole. I was afraid of not getting proper air circulation in the tower with such a small access hole. I went with the copper tubing method and used 3/4" ID copper tube which will fit 2, 7/16" od beer lines through it. My guess is your pulling all the cold air from the fridge into the uninsulated and warm tower which warms the air but it cannot recirculate back into the fridge, which is being counterproductive. Does your compressor run more with the fan on?
I haven't gotten mine completely done yet, but with 2 kegs full of sanitizer and a 5# CO2 bottle in the fridge it will get down to 42 degrees F with the thermostat set to max. I did the gross screw adjustment (two full turns clockwise) with the thermostat and it will get well below freezing with little effort. I had to back the gross adjustment screw a turn and a half counter clockwise to keep things from freezing. I'm waiting on an 80mm computer fan to come in that I'm wiring to a cell phone charger before I fine tune the thermostat. This will just circulate the air in the fridge itself. The copper will,hopefully, take care of cooling the tower. Fan should be in any day, I'll let you know how it works out.
Here is how I'm cooling my tower.
I ran the cell phone charger cord for the fan power up through the drain hole.