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Many late model GM vehicles do not have a coolant drain plug, depending on radiator manufacturer.
If your radiator does have a drain, it will be located on the bottom of the radiator tank on the opposite side of the vehicle from the lower radiator hose. That is, if the lower radiator hose is on the passenger side of the car, then the drain plug will be on the driver's side of the vehicle.
Even if it does have a drain, they are often all but impossible to reach.
To drain the cooling system, the easiest method is to loosen the lower radiator hose and remove one end; this works quite well and is the method we use in our shop even for vehicles that are equipped with a drain.
I hope this is helpful; if so, an accept would be most appreciated! thanks!