I rebuilt the engine compartment wiring harness on a '85 Volvo GLE once. Volvo used bad wire for several years back in the 80's and after a few years (read as "after the factory warranty runs out") the insulation on everything (except the black wires - black is ground, of course) would crumble away and thing would start shorting out. Volvo wanted $500 at the time for a new harness (just the part... labor extra). So, I took the old harness out (labeling everything) and laid it out on a 4x8 sheet of plywood and secured it in place with house electrical staples. I had access to a large amount of MTW (Machine Tool Wire - oil and gas resistant) in various gauges and colors, so I just replaced one wire at a time, matching the gauge and using the closest color I had, cutting to length and soldering to an inch of the existing wires at connectors and using heat shrink tubing to insulate it. I put new wire loom on it when I was done and just put it back in. Everything fit and worked perfectly and as far as I know, that car is still running with it. A little time consuming, but not a difficult job.
'93 Limited (sold), '96 Laredo