1999 Dodge Dakota Poor heater performance

My heater has never worked properly. It will throw a little heat however not enough to defrost the windshield let alone warm up the vehicle. It throws less and less heat the colder it gets. I have replaced the thermostat twice, had a cooling system flush and put in new antifreeze in. Both hoses going into/out of heater core are hot. I was told one should be hot and the other cool. Is this probably a defective heater core? Or is there something else I should try first before replacing the heater core? Thanks.