Transmission plumbing question

It doesn't matter which way it runs through the radiator but if you're going to run the cooler as an auxilliary unit then it should pass through it last before returning to the tranny. That way the rad does the initial cooling, which presumeably would be hotter than the external cooler. That's what I see the manufacturers recommend and have always done myself. Fitting at the front of the trans is the feed line, rear is return.