Which trani line is return line? 87 Caprice

I've got a 700r4 and am wondering which trani line is the return line? The line connected to the upper or the lower part of radiator?
I picked up a trani cooler off a junkyard car and want to flush it out to avoid the possibility of contamination.
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Typically, the line at the upper end of the radiator is the return to the transmission You should also verify that the lines at the transmission are connected correctly. The TH700-R4 is essentially the same as all other longitudal (rear drive) HydraMatic units except the TH200-4R.

For all longitudal units except the TH200-4R the lower oil port on the side of the transmission is the supply from the pump (flowing to the bottom of the radiator oil cooler). This includes the TH350 and 350C, TH375, TH200C, TH400, TH700-R4, 4L60/4L60E, 4L80E, and ST300/Jetaway. The TH200-4Rs are just the opposite.