you can use it either way, i use to have it hooked up through the radiator, and now i have it straight to the cooler. I would say as far as cooling goes, there are arguements for both, but the temp range really shouldnt make a huge difference from either way according to what ive read on this forum from a few people who have temp gauges. but check this out, i guess i had a small leak in the radiator, and water got into the trans fluid, and caused slipping and other problems, so i might have to say it may be a little better and easier (install & $$ wise) to just run it straight to the trans cooler. its easier to run it that way, and if you ever get a leak in the radiator, you wont have to worry about water mixing with your tran fluid and ruining your clutches and such.