Tranny fluid leaking at radiator

I have just had my tranny rebuilt (2003 Sable), and now I have a leak at the radiator. The tranny rebuild is still under warranty, but the mechanic says this is not covered under the warranty since it is not part of the transmission. He says he can fix it by replacing the radiator for about $500.

I have seen radiator coolers at autoparts sites on the net for $25-$50. Could I just bypass where the fluid goes through the radiator and install an aftermarket cooler? If so, where does the aftermarket cooler go (not much room). Any help would be appreciated!...

This is what I would do. Take it somewhere else to have them verify that it is a bad radiator or just a simple fix leak. If its the rad, I would have it replaced. If you put in the stand alone cooler and the fluid temp gets too high, hows the fan going to know to come on to pull air through the cooler? A hot running tranny wont last long!

Also, an after market cooler could void the warrentee for the work already done even if the existing problem turns out to be the fault of the mechanic that did the original work. I would attempt to cure the leak and if that fails, replace the radiator. If you are sure the radiator didn't leak fluid before transwork was done, then he needs to prove that he didn't touch it or damage it by pulling on a trans hose line to hard. The location of those lines on my sable radiator are difficult to access, I have dealt with them a few times through the years, I am glad I have smaller hands than the average joe.

Exactly where is it leaking from? One of the screw-in fittings or the transmission line(s) themselves?