Is the "wiring harness" just the wires going from pots/jack to pups? I can't see new wires making a difference... Also, are there "better" pots?
The wires won't make a huge difference. Some are thicker than others. That might add to durability, and there are those who feel thicker wiring may enhance things, but I don't really buy it. I'm not an electrical engineer or anything though.
Pots (potentiometers), however, DO make a difference. There are variables in quality and also a lot of internet discussion about 250 vs 500 pots, which is better for which bass or hoped-for sound, etc.
I am seriously considering bypassing the tone pot on at least one of my basses and wiring direct to the jack. I don't use the onboard tone knob much anyway, and some people swear this mod opens up the sound in a nice way (because there are fewer "thingies" the wiring runs through.
Again, I'm not a tech. Some of this "frankensteining" is going to be a learning process for me as well. I am pretty handy with a soldering iron and have disassembled wiring on many guitars and basses in order to do refinishes, but I've never done an actual pickup or wiring swap. So it remains to be seen how competent I may be at this...