Steve;

...pretty skimpy on details, but 1800 Wiring Diagram such as this ( http://www.sw-em.com/1800%20Wiring%20Diagram.jpg ) does not give wiring gauge info, but 122 WD does give gauge info (in cm2) see (http://www.sw-em.com/122S%20Wiring%20Diagram.jpg ). You can use this as a guide.

I just wonder why you would need gauge info if "refurbishing and reinstalling"? If you have the old harness and wire in front of you, you can easily measure the gauge...simply maintain gauge when making repairs or adding extensions...and if you are installing a new harness, constructor of harness has (presumably and hopefully) used a correct and appropriate gauge for each wire, so what would you need gauge info for?

As far as routing, if you removed old harness yourself, and didn't take notes and pix, you might remember to get this info next time...maybe you can get some brave soul to do the contortions necessary under the dash to get this info for you...

Cheers