99 Cadillac Deville

Sounds like you might have a leaking head gasket although that isn't real common on your car. There are three different tests that can be performed. One is a pressure test that puts the system into 15 pounds of pressure, then you search for the source of the leak. The second one is to add a small bottle of dye to the coolant, then you search with a black light. The leak will show up as a bright yellow stain that you can follow to the source of the leak. The third test involves drawing air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are present from a leaking head gasket, the liquid will turn bright yellow.

You will need to check the a/c case drain. Its right below the evaporator for signs of coolant. Heatercore may be leaking. Gl