Maybe its had a prang as well as an earlier engine change and the repair shop put it back together a bit differently to factory (as Bill R alludes to). If it has had a front-ender you never know they may have opted for a transmission cooler mounted inside the radiator instead- it seems weird as the pipes would have to change and all, but I've seen repair shops do weirder things... but if it is original, you simply have to isolate the leak through observation, none of us can give you a magic spell or anything else to recite on that... have you checked the transimssion fluid level according to the manual's procedure?
A lot of people overfill their trannies as they 'know better' that to follow it to the letter...
Yours is a ZF5HP18, a very nice transmission so if you have to pull it out and rebuild or replace that is an option. A rebuild is a good idea if you want to keep the car, replacements could be short on life too- but the are quicker and cheaper to do. Perhaps you could donate the old tranny to someone local who might like to rebuild it...