I have a 2000 Ford zx2 and my heater is not working. I checked my thermostat and replaced it and I still get cold air.

Are both heater hoses going to the fire wall warm?

Does the blower motor work ? If yes then it could be the little valve that is inline with the heater hose,I don't no the proper name for this part but it's a valve that opens and closes when the heat is turned up on the dash control.This mite not be working or a vacume line that operates the valve could be malfunctioning.

Try to see under the dash if you can see this valve,turn up the heat and see if the valve opens and closes.