I have been wondering about that flow direction. I have my add-on tranny cooler mounted and the hoses routed but haven't hooked it up yet. I wanted to find which direction the fluid flowed to make the decision on how I hooked it up. Thanks Todd.
My experience has been that the flow first to the aux cooler, and then to the radiator. The radiator is to maintain the proper temp (not let it get too cool). YMMV.