Never worked on a BJ8 so don't know what removing the console involves, but agree it would need to be out of the way. I think once you get behind the console, all 100-6 and 3000 heater control panels are the same.
If you are only changing the fan motor switch, you should not have to remove the heater itself.
Remove the 'fan boost' knob (has a set screw going up thru the bottom that you need to remove to take the knob off). The control panel is bolted to the dash with two bolts passing up thru the control assembly and some spacers with nuts on the top (inside the dash panel). There are two spacers on each bolt, so note the locations as you take it apart. Once you have the two bolts removed, the whole assembly will hang down a bit and you can see where things are attached & how to access the water cable & other details.
You will have to disconnect the water valve cable at the control lever and unplug the wires from bullet connectors and remove the lever and switch from the Bakelite panel. Not certain but I think you should be able to leave the air control cables connected and the control panel hanging loose in the car.
If you bought a 'lever and switch assembly', putting it back together is simply reverse of disassembly. If you only bought the switch, it is riveted onto the lever assembly and the wires may be soldered so, you will need to drill out the rivets and replace - then reassemble. Make sure you adjust the water control valve cable to be sure it fully shuts off water flow when closed.
It's a PITA but doable.
Dave