My heater control selector is not working. How do I fix.

Good point could be a plugged heater core. But fan speed is def resistor .

Agree, blower resistor likely fan speed culprit .. Run engine until it's at normal operating temperature according to the dash gauge .. Grasp both heater hoses at the firewall .. Both hoses HOT? .. IF coolant is flowing through the heater core as it should then BOTH heater hoses should be as hot as the TOP radiator hose!