Hopefully that's a long way off - and most stuff is fairly obvious and if not just a multimeter and a long fly lead.
What's hard is when there's the old and the repair and the accessory and the mystery all muddled in together - and this is CLEAN.
First job I got done was a rewire - a HEAP came out - and once John Savage had put in a new loom it wasn't touched again in my ownership and as far as I know since.
I expect if it was me I'd start with a heap of colours under the bonnet and some multicore down the sill and then run out of something and think "if I make a note of where it doesn't match up it will be ok" and of course then someone would wonder why the wire starts green one end and comes out purple the other

David