I actually went ahead and added a twin transmission cooler setup in my G6 not too long ago, Came out pretty well and well worth it! My transmission LOVES it. I wrote a HOW-TO here: https://www.g6ownersclub.com/forum/sh...ad.php?t=12261
The stock trans lines are 3/8"s at the rubber hoses (where I tapped into) All trans cooler kits come with everything you need except for the barbed fittings to tap into the vehicles trans lines..
I would say install the trans cooler AFTER the radiator so it still gets a chance to warm up in the cooler weather.
I went with two Light Duty coolers, you can do the same, or go with one, or go with a medium duty if you want.
There seem to be two different kinds...
I used this kind:
Mainly because it's still a solid 3/8" line that just passes back and forth through the cooler.
But this style cooler:
This "Stacked fin type" Is more efficient but may get it too cold IMO. So that's why I went with the less efficient style, So it cooled it but didn't over cool it haha.
But please check out that How-To and feel free to ask any questions! I'm more than willing to help out!