The drain plug is probably on the left (driver) side. If you remove the air intake box by loosening the screw that tightens the intake rubber hose to the throttle body, you can lift the air box containing the air filter and remove the plastic mount. I'm going by my '94 Eldorado so your 99 Seville may have a different set up. Once the air box is removed, look under the radiator hose. You will likely see a plug where you can drain the radiator. When you refill, you should use 50/50 with distilled water.