What would cause the internal vents to blow cold air when the heat is on?

check both heater hoses under the hood with engine at running temp. if only one hose is hot your heater core is restricted( try flushing it out),m low coolant will also cause no heat since the heat comes from the hot coolkant in the heater core. if both hoaes are hot chances are the blend air door in the interior heat unit is faulty or climate control is not calling for heat.