Step one, wash engine compartment. Step two, watch for signs of external leaking. Step three, read the GM litigation web-site about Dex-Cool lawsuits. Step four, Go to GM dealer armed with information about this. Step five, in the future, use only Mercedes Benz coolant in this car to prevent cooling system damage for 150,000 miles.
where is it leaking from. when your car is cold, open the radiator cap and start the car. let the car warm up. when the car is warm, look at the water in the radiator and see if it is moving. if it is not, your new water pump is not working. is the belt hooked up to it?. if it is working than something else is plugging your water from circulating your engine.
where is the water leaking from? If from the radiator, replace it. If you can't find the leak, check your oil and make sure it isn't leaking into it. If it is overheating, you may have blown a gasket.
If your radiator isn't clogged up , check to see if your thermostat is hanging up, may not be opening when it should causing your engine to run hot and boil over onto the ground. You could have a blown head gasket ,check your oil and see if you have coolant inside of the oil,it will look milky in colour, you might also unscrew the oil led and look on the bottom side of it to see if there's coolant under it.