So, I carefully examined every wire on the harness and miraculously, all the copper threads were intact - only the insulation was eaten away. I even separated all the individual wires, turned the key on to double check this was in fact the case and low and behold, all the lights went off on the dash. Because of this, I decided to not make things worse and just carefully wrap each one of them back up with some automotive fabric tape. Here's a shot of what the harness looked like before.
And, here's what it looks like now.
I'm thinking this should hold up well enough and if not, I can go through and do a proper fix.
In the mean time, I found what looks like the most logical way up in to the dash for the mice. Essentially, they would crawl up a space located between the firewall and the kick panel and along where split conduit and the wiring harness travels up into the dash. This is something I'd going to have to plug up or find a way to limit acess.
♂lol