1999 Epedition Mopar radiator failure

'99 Ford Exp. 4.6 V8 with tow package. I have replaced the radiator 3 times and it has a reoccuring leak in the same spot every time. Pro Max has done the work and it has a lifetime warrenty but they still charge labor costs. Should they be charging this??? Why is the leak occuring in the same spot with new radiators??? I have changed the pressure cap. I also had it checked by another mechanic and he said the engine is fine. Head gasket is ok also. Please help. no over heating just leaking. :confused: