"Would you consider offering a harness rebuild service?" It wouldn't be a bad idea, but for now, I will stick with my day job, plus I live in Brazil, a little too far ! LOL.
In reference to your questions, I used 3 different guages of wire 14,16,18, the original harness uses these sizes and a 12 gauge for one of the grounds, I didn't touch the MAF sensor wire and a couple others, for they were in good shape, the critical ones are the ones that ride next to the engine block, like injectors, coil, etc.. there are a few connectors that you can open and solder the new wire at the terminal. There is one sensor that uses 8 wires,I beleive it's the idle speed sensor, I had to cut it open with a Dremel tool, in order to get REAL close to the terminals, the shielding on these wires were cracked all the way to the connector, after the repair, I rapped the wires with a special rubber tape, to build some volume, so that the clip that holds the connector to the sensor could grab.
One of the difficulties I had, was in disassembling the harness at a junction box located on the left fender (picture below), DO NOT force it ! it comes a part easily, once you figure it out.
I bought all the wires, shrink tubing, special tapes from one source
http://www.waytekwire.com/
TK, Ed
