The sealant tablets go into the lower radiator hose on the northstar system. Here is the shop manual warning for the Northstar:
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This engine uses DEX-COOLŪ and GM coolant supplement (sealant) P/N 3634621 specifically designed for use in aluminum engines. Failure to use the engine coolant supplement (sealant) and the approved coolant antifreeze could result in major engine damage. When refilling the cooling system, add three pellets of the engine coolant supplement sealant GM P/N 3634621 to the lower radiator hose
More info here:
http://caddyinfo.netgetgoing.com/how...tconfusion.htm