How do you change a heater core on 98 explorer?

The dash panel has to be removed along with the steering wheel and steering column to access the plenum box. The plenum box then has to be removed to get to the heater core. This requires evacuating the AC system, draining the radiator, disconnecting all lines and removing the box. Replacing the heater core is simple when you get to this point.

If the problem is lack of heat(not a physical leak), the problem is more likely with the blend doors on the Explorer. This is a very common fail mode. HeaterTreater has a line of products to address the issue at a low cost. Follow my user name to my Bio Page for more information. ford4me