What you said is how its done. You get new looms, lots of wire and extend them one at a time so you can't cross anything. Then loom the extensions and run them through the inner fender. And I've seen some brilliant individuals doing it and posting progress pics using butt splices... plz solder/heatshrink lol.
I have not attempted it although I know I could do it. Looks like a total pain in the taint and a vanity thing more than a functional thing honestly.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.