Climate control

That is an indication coolant is not moving through the heater core to heat up the air.

The most likely cause would be a restricted heater core. Disconnect both hose and back flush to get any restriction out of the heater core. Check hoses for clogging due to kinking etc.

That would depend on how cold the weather is. You were able to replace the thermostat and radiator which is a more time consuming job.