In my car, the top line is the one that sends fluid from the stock cooler to the tranny.
I bought a tranny cooler, one with a fan, a long time ago, but I've never installed it. I'm also thinking of installing a tranny fluid temp gauge, which I'd like to install first.
There are lots of threads about this. Just do a search in this website's forums, and on
www.corral.net forums.