How do you remove the dash from a 1998 Lincoln Navigator to replace the heater core?

You do not have to remove the dash. You do however have to remove enough of the ductwork to allow access to the heater core. This is almost never an easy job and as I have not replaced one on a Navigator, I cannot tell you exactly what to remove. Just remove what is necessary to get to the core. This is why the Pros charge so much to replace a heater core. Just remember exactly how the parts cam off and how they go back on. Good Luck.