On a scale of one to impossible, how possible is it to replace in its entirety the Interior wiring harness?

Scot

It is very possible. Just a lot of work. You need to take out a good part of the dash and interior.

I think it is very rewarding though..

when i redid my green machine i took the dash out and ordered all new fasteners and a new harness .. I was able to clean everything under the dash and organize the wiring better than when it rolled off the assembly line.

(get various sizes of colored wire ties to make it very neat and understandable)

this is definitely a winter project though..

you will need a good wiring diagram for your year before you start... another thing that helps is if instead of REMOVING the old wiring properly if you cut it off and leave stubs connected to everything.. this way youll have another refernce as to where the pink wire goes and where the yellow goes..

i also have a panduit wire labeler that i use if there is any doubt.. or for color replication.

and the most critical rules for rewiring.. measure THRICE cut once and make sure you have it routed properly before you measure. ie make sure before you cut that you mock it up to fit everything back in..

im planning to do it this winter when i do my frame off..

you need to be the kind of person who can take a car and turn it in to a pile of parts and not get overwhelmed by the magnitude of reassmebly though..

I was spared the agony of the interior harness, i did have to replace the engine harnesses though. It isn't as bad as you might think. A suggestion would be to leave as much of the wiring in tact and take some digital photos during the removal. It will make a great reference when installing the new.
I had my entire interior out to re dye it, so all of my wiring was exposed. A complete harness should be cut to the proper lengths, have the correct colors and ends. That also makes it easier to re-install. The gauges and dash cluster are stamped which color plugs into what hole, also a big help.
One issue I did come upon was routing, just make sure thw wires when run, do not interfere with moving parts down under. I have to run my speedo cable again because I wasn't paying attention when I put the clutch pedal back, no big deal, but I should have paid closer attention.
Good luck and keep us posted.