What is the simplest wiring harness for a race car
the one you build yourself, no wasted wiring or extra wires to take care of if you don't know direct current formula for the size wire to use on each circuit, learn it
I agree, build it yourself for your specific situation.
Use quality parts. Not auto part store stuff. Read and make sure you understand grounds and good construction techniques. Use Weatherpack at a minimum. Deutsch if you want the good stuff.
I like ARC stuff, but prefer the 3100 panel for most applications. Or build your own with good rocker switches.