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My company builds large industrial engines and wiring harnesses are our biggest engine issue- specifically the quality. And so working in service we are constantly troubleshooting “wiring harness” issues. Within the harness the most frequent are improper crimping, improperly seated pins and sockets, poor control (wide range of quality) of connectors, and mixed up pin/socket location within connectors. The ability to test the harnesses is critical, but can’t replicate functionality while under vibration (a running engine). I look at harnesses and think how difficult they can be to manufacture..Not to discourage you as I expect you could fill a quality...

Great idea, IMO. I can provide a bit of insight on this as I have had wiring experience (various race cars/Formula SAE) and someone I tangentially know spun up his own wiring company catering to a very specific make/model and has since branched out. As with anything else, the big up front cost is the tooling. You'll need multiple crimping tools depending upon what types of connectors you are using, several spools of quality wire with the right colors/tracers if you want to follow OE specs. In order to really nail down your QC, you need some sort of jig...