On a 2005 Pontiac Grand Prix 3.8 Non-Supercharged and Automatic Transmission.... which of the cooler lines is the return line?

Cooler inlet hose is the horizontal fitting in the trans to the lower radiator and the cooler outlet hose is the vertical fitting in the trans to the upper fitting in the radiator. Fluid goes into the cooler at the bottom and returns to the trans through the line connected to the upper radiator fitting.

Thank you...I always seem to get that backwards....

When in doubt, disconnect the upper line, run the engine and see where the fluid goes.

All over the floor.... LOL