Before flushing and with engine at normal operating temp. check both heater hoses to see if they are hot---they should be. If one is hot and the other is not, suspect a clogged heater core.
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Disconnect both heater core hoses at the firewall. use a water hose to run water (at a high rate of speed) through the heater core. doesn't matter which hose you use as the inlet or outlet during flusing. I usually flush through both hoses (one at a time).