Hello all, I was sent here by WilliamE because the other forum I posted on could not answer my questions. I am really hoping for some help with transmission cooler line replacement. My 98 Grand Prix GTP is leaking transmission fluid from the lower line (or fitting/o-ring) on the radiator. The fitting on the radiator is pretty badly rusted, as is the line. I have read that the O-ring in the fitting has a tendency to leak so I was planning on replacing the fitting, o-ring, and the line. Also, the fittings are pretty cheap so I figure I may as well do it while I am in there. The problem is, the hayne'* manual doesn't cover how to perform the repair, and I can't see the clip that holds the line to the fitting due to the rust. I do have the gm disconnect tool (little 3/4 circle device that has a fin on each end to squeeze together), which is the only oddball tool I have seen any reference to. Any advice you folks can give as to which fittings I need and tips on how to take care of the problem would be awesome =) Pictures or diagrams would be great too. Thanks in advance!

Or would I just be better off adding an aftermarket trans cooler?

IF you can get at the clip you can use a pick to fish the clip out and then just pull the line out at the radiator or transmission.
All so don't loosen or take both fittings out of the radiator at THE SAME TIME!!!
The inner tank will fall to the bottom of the radiator making it about impossible
to put the new fittings in with out removing the radiator!!!!!

Welcome llama, I am sure some of the guys here can help you, I am not sure how different my 2006 GT is, but soon I'll have the Factory Service Manual for it, I am pretty sure they should be the same, if so I'll scan the pages on how to do the job for you, I should have the FSM in a few days..
If not I am sure somebody will be able to help you, Karl has already provided some good tips!

I am really happy he mentioned the bit about removing both fittings at once. I watched a youtube video the other day on how to remove the clip but I didn't hear the part about him replacing the radiator because the mounts for the line fittings fell into the radiator =)

You would not have to replace the radiator but most likely you would have to pull the radiator out and shake the inner cooler back in place so you could thread the fittings back in.
That'* why just remove 1 fitting at a time if your replacing the cooler fittings.