i got my trans cooler installed over the weekend. Ran it in series with the radiator one...so it comes from the trans, into the new air cooler unit (mounted low behind the front bumper and air dam) then up into the radiator unit...
Just an FYI for future generations to save them the mess, on the 99 suburban the line from the trans the bottom input into the radiator is the HOT supply fro the trans...then it flows upward thru the rad and returns via the upper line back e trans. It was rather messy finding this out.

the routing and placement of the new cooling unit was figured that the air may overcool the fluid but the rad will bring it back to a decent temp..and if it happens to overcool then it will aid in the engine cooling.