I am the tech manager at Prestone coolants. Sounds like a tough one to find. I use a dye in the coolant, you add it, run the engine for a while then use a black light to see the leak. You can buy the kits on ebay with dye for engine oil, trans oil, ac and coolant with the 12v blacklight for around 30 bucks. they work very well for finding difficult coolant leaks. Good luck and let us know if we can provide further information.