I have replaced my thermostat and heating core why is my heat blowing cold in my 99 Ford Expedition??

The coolant system might have trapped air impeding flow, try "burping" the radiator a couple times. (google how to do that for your make/model). You can check if the two heater hoses to the heater core feel like they have pressure and hot coolant in them when the truck is running as well.

If vehicle has cap on over flow tank bleed air from system like this. Remove cap fill with anti freeze to full mark leave cap off start vehicle and let idle and reach operating temp anti freeze will start to fill up over flow tank this is normal once t-stat starts to open anti freeze level will drop in over flow tank fill back up to full mark then put cap back on. All air will be removed from cooling system.