ford installed a goofy bypass hose between the inlet and outlet hoses going to the heater core. the least little bit of restriction in the core will cause the coolant to bypass the core. pinch that hose off and run the engine a while. sometimes, that will blow the restriction on thru and the heater will work. later, remove what you have pinching off the hose and see if the heater will work without the bypass restricted. we have had to leave it pinched off to eliminate replacing the core.