Radiator drain on the left side underneath. Remove the underside shield. Red in color. I would not mess with trying to open a block drain. Disconnect the hoses, remove the thermostat and flush the block with a hose with the engine running. When it comes out clear, you're all set. You might want to try the citric acid flush thing as well before you add the coolant. The block drain is, if I can remember, on the driver's side below the intake manifold. Flushing with hose is easier and does a good job. There are other posts on this subject. Do a search on flush or coolant system flushing.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.