Factory Trans Cooler-Flush-Which line on cooler?

[May-39]

I did search "flush" in the search function first, did not find answer.

I have a new to me 99 obs with the trans cooler package (KNP). It has the plate trans cooler in front of radiator.
Planning on changing trans fluid/filter.
Which line on the cooler would I pull/tap into to connect to plastic line for flush?

I apologize in advance if this seems obvious, this is my first 4l60e vehicle. I really don't want to screw this up and hurt this trans.

 

[SunlitComet]

you actually have two coolers. one in the radiator as well. the out let of tranny should be the line that enters the radiator first and connect to the upper rear port of tranny. it goes from there to radiator to external cooler then back to tranny in the forward lower port. so if you choose either flush direction you now know what line to use.