I have a 2004 Suburban and I'm going to drop the pan and change the trans fluid and filter. I also want to change as much fluid as possible. Can somebody tell me which trans line at the radiator comes from the transmission and which line goes back to the transmission.

 

Once the truck is warmed up the hotter one is going to be your IN and the cooler one will be OUT.

 

The feed TO the radiator is usually the bottom line. The return is the top line most of the time.