2000 F250SD 4x4 5.4 auto Heater not getting warm. How do I get to door under dash to check if it is operating?

Find your heater core/ac evaporator and remove the cover. Try changing the controller from heat to cool and see if the door opens/closes. Sometimes the foam insulation on these comes loose and will cause the door to become stuck. If door does not move and it's not stuck you possibly have a bad controller.

you cannot remove the dash and take off the cover and change the controller. at that point the dash would be removed and nothing would be plugged in. check your coolant level first, if it is low it will not make good heat. the temp door actuator is electric on those and is on the bottom of the box. they go bad often. dash has to come out to replace.