What is the best way to go about fixing this mess? There doesn't seem to be a way for me to pull whats left of the harness down from under the dash without removing the entire dash, is that correct? This was all like this when I bought it, unfortunately.

 

Id try a slicing of the black protective tube for the wiring and that tape. Open the tubing to pull more length of wiring. Use shrink wrap and cut enough of the way behind the corrosion to ensure proper connection, if possible. They sell connectors with long lengths of wires. You are in a tight spot there. Mine was a similar case, but in my Wrangler.