a couple years ago i had to replace the blend door on my 97 e150 and i know i had to remove the heater core.

book instructions for heater core removal

1. Working in the engine compartment, remove and plug heater inlet and outlet hoses from the heater core. Note: Some models use quick disconnect couplings on the heater hoses and require coupling tools. My Note: i just cut the heater core hoses as i did not have the tool to remove couplings but what i did have was aviation snips and plenty of hose clamps.

2. Remove the hush panel along the lower edge of the instrument panel on the passenger side of the vehicle by gently prying it out of its retainers

3. Remove the heater core cover screws from both sides of the heater case.

4. Remove the heater core cover, being careful not to tear any sealing material that may be in place.

5. Remove the heater core.

the blend door removal was all blind as the only thing i could see was the front of my dash, in my next life i would like to have eyes on the end of each finger.

good luck i hope you get it working