I used Simple Green and Fast Orange hand cleaner. The longest part was removing all the tape and old loom. The cleaning part actually went pretty fast.
I have a utility sink. I threw a stripped harness segment in the sink, sprayed it down with Simple Green, let it sit for a while, squirted a glob of Fast Orange in my hand, and pulled each wire through my hand. Rinse and you're done.
The WD-40 should penetrate the dirt well, but the oily residue it leaves could make dust stick to the wires.