Suprised no one picked up on this yet. Do NOT "Flush" the radiator per se - simply unhook the LOWER hose and let as much coolant as possible drain completely. Probably a gallon or two will come out - then measure a 50/50 mix of about the same amount of clean, drinkable water and Dexcool, and refill. (After hooking the lower hose back up.. LOL)
If you look to the top of the forum home page, you'll see a link for "Cadillac Technical Archive" - the detailed instructions will be found therein.