I'm new to the forum but not automotive work and I'm having a brain cramp, hopefully some one can help. I am installing an external trans cooler on my 1993 K1500 Suburban and I can't remember which on the 2 trans lines going to the radiator to cut. I want to cut into the one coming out of the radiator going back to the trans and I believe it is the bottom one out of the radiator but like I said, I'm having a brain cramp. Can some one help please?

 

Not sure just which line (location) you need to find, but, seems like tracing them out should be possible.
The route should be from the Tranny, to Cooler, to Radiator Capsule, then back to Tranny. This gets the super hot Fluid to the Cooler first, while still maintaining the ability to warm the Fluid up when cold.