Quote:

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"C should be the supply
D should be the return"

Meaning the coolant flows FROM the engine at "C" to the heater core and returns to the engine at "D"?
Have Igot that right?

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Correct. If you don't remove the thermostat then the best way to flush the system is to back flush by connecting the hose to the "C" fitting and remove the bottom hose (engine off). You can flush the radiator separately. Unless you pull the drain plugs in the block (which is nearly impossible if they are in there forever), you will never get all the crud out.