Glad to see some responses from folks that have actually worked on repairing harnesses. Your responces reflect WHY I got out of doing this work. No one is willing to pay shop rates for 30-40 hours of actual work for "a simple harness".
I was selling harnesses at near my cost just because I loved being able to put a smile on someones face when they opened the box. I am now less than two years from retirement and my full time day job is just eating up too much of my time.
Now that said, I have worked on WAY worse harnesses than what I see there. That harness can be easily repaired!
Some suggestions...
1. Use something like Simple Green, 409, or Krud Cutter to clean the harness BEFORE you start work so you can easily see the wire colors. Acetone will also remove the really bad stuff (and some paint overspray) but be careful with that as it will melt a lot of plastics.
2. Once you have it clean, start on the drivers side and make a list of everything you need to repair/replace. Do not skip this step as it is so easy to forget something you spotted when working on it. Keep that list handy so you can refer and add to it as you work on it. Order your parts and wire based on that list.
3. Work on ONE circuit at a time!!! Due to Toyota using similiar wire colors it is so easy to get confused when tracing and repairing wires. The schematic and a good multi-meter is your friend.
4. When replacing a section of wire inside the harness, use splice crimps to splice in the repair section. These are what Toyota used to make splices. Slide heat shrink onto the repair section before crimping.
5. When replaing a section of wire that comes out of the harness, make the splice INSIDE the harness so it is not visible.
6. Find out if the firewall grommets are availabel for your truck, and if so buy them. You can put them on the harness while you have it apart. Otherwise you will have to cut them to install them after the fact.
7. Make sure you get a good set of open barrel crimpers for crimping the terminals onto your wires. Try ToyotaMatt at VintageTEQ.com
8. Anything that is made of brass (terminals, fuse block clips) or copper (like old corroded wire you are trying to solder or crimp a terminal on) can be easily restored to almost new by soaking them in white vinegar with some salt added for a few minutes. RINSE WELL to remove this concoction!
9. Replace all the fuses with new. Don't think about it, just do it! I have wasted a lot of time because a fuse "looked" good. Use marine fuses made with stainless steel end caps.
10. Switches: Most of your switches are probably good. If however they are not good when you start testing don't panic thinking you need new ones. These switches are well built and will just need taken apart, contacts cleaned, and regreased. Search for switch cleaning on here to see what to do.
11. Once you have the repairs done, but BEFORE you tape it back up, test EVERY circuit with the switches plugged in and the lights plugged in. This one I cannot stress enough! You can rig up some test lights with some old bulbs, some wire and some terminals. Just move them to the plug you need to check.
12. Grounds: When testing YOU MUST HAVE THE GROUND WIRES GROUNDED TO THE BATTERY NEGATIVE. Most harnesses have more than one ground. The ground wire color on all FJ series trucks is White/Black. For testing you can jumper them together.
13. Fusible link: DO NOT test without the fusible link(s) or some other fuse between the battery and the harness! One slip and you can melt all your hard work. Because I "slipped" on a customers harness, I had to rebuild the charge circuit on their harness. Due to that I came up with a temparary fusible link that uses a fast blow fuse instead of a slow blow link. It plugs into the same place the fusible link does. You can also just put an inline 15 amp fuse between the battery + and the fusible link to protect all circuits as you test. Just test one thing at a time.
14. Taping it back up. You have three choices: Use electrical tape ( not a good choice as it will make a sticky mess when subjected to under hood heat), use Tesa tape, the kind used on modern vehicles, but still an adhesive based tape or use non-adhesive harness tape that sticks to itself, but not the wires. I use the non-adhesive harness tape on all my harness work. A full harness will take about 3 rolls depending on their length.
Hope that helps you with this project. That is one beautiful FJ45!