I've never had to replace them but looking at the 3rd gen service manual, there's quite a bit of stuff to remove just to get in there. Most of the upper and lower HVAC housing, which includes the heater core, A/C evaporator, blend air and mode air doors, air door actuators, air distribution manifold and the rest resides behind your dash face, basically all the way from the driver to passenger sides of the cabin.
You start under the hood by draining the coolant system, removing the heater hoses and the PCM. You remove 4 bolts from the firewall, then move inside.
At this point you have to remove the instrument panel, the audio, the glovebox and various harnesses that can be unclipped and moved out of the way.
3 more bolts and the assembly is ready to pull out.
How good are ya at laying on your back on the floor of the cab and unbolting stuff?