How EFI wiring harness relates to other electrical systems

What i'm grasping from the courses that i have purchased are that i'm going to learn how to build an EFI wiring harness and thats fine. The part that seems to be missing for me is the rest of the wiring in the car (headlights, horn, indicators, fog lights etc etc). My build is a road going kit car and i have a donor loom from an MX5 which i was going to use. However i would guess almost 70% of the wiring is not required so i want to build a loom from scratch. So if i build an EFI wiring harness should i complete that in a standalone format and then build a separate harness the rest of the cars needs or should i be doing it all together? Bear in mind i am a novice here and whilst i have found mechanical stuff like rebuilding an engine easy enough i'm concerned i am lacking serious knowledge on knowing where to start and even how to plan my build. Thanks

In general high current device supply (headlights, fans, pumps) is best done separate from the ECU harness. The ECU harness goes to the relays, so where the two harnesses connect it at the relay panel, or maybe at the dash switch area.

I had the exact same questions going thru my head when I first undertook the wiring course and since then once you learn all the fundamentals and understand how the electrical circuits work you will be able to work thru the wiring diagram from the MX5 to determine what you require and what you don't. There is also a very good webinar what talks thru some of the other circuits within a competition car such as head lights, indicators etc in the webinars section.

Once you wrap your head around the EFI side of things post up some specific questions and I have found most like David are able to help.