There are several ways. I have a simliar setup. I only have insulation in the tower and no insulation between the top of the fridge and start of the tower. My first pour is foamy and its fine after that.
You can use the same methods seen around here in various posts. Some people wire a PC fan to blow cold air up the tower. The other popular way is passive, you just run the beverage line through a copper tube up the tower. The copper tube has a good amount of length that is in the kegerator either by turning 90 degrees and runnning along the top of the inside of the kegerator or straight done in between the kegs.