Which holes do I use to flush heater core

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.
Post results and let me know if more assistance is needed.

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).