Quite frankly, having an OEM wiring diagram might do more harm than good. Why? 'Cuz you'll be spending unnecessary effort comparing what you do have to what it was supposed to be.
I have been there, a few times.
I spent a week of almost full time wiring diagrams and analysis of what was there based on our OEM manual wiring diagram and the one my PO gave me. I found little changed from the OEM, but LOTS of errors in HIS, both of what it showed vs. what was really there and what he claimed was there and simply wasn't!
Completely wasted time.
The best bang for my buck was starting a brand new diagram of "This is what is there."
And then, only AFTER I did that could I compare it to what I wanted to do, which I developed NOT from what was there but from what was right and necessary. A real design.
Then I figured out whether whatever I wanted to have finally was worth either changing and modifying what was there or whether I had to rip sections out and start over.
Make a design of what you want.
Document what you have.
Make comparisons and decide the physical things you need to do.
Forget some pie-in-the-sky OEM "what could have beens."
I say this from a 40 year engineering career where a lot of my work was investigating old buildings, analyzing them for reuse of various kinds, and figuring out what it would take to make it what my clients wanted. The very last thing I ever asked for were the oringinal plans. Why? Besides the fact that few had them, it was almost guaranteed they hadn't been kept up to date anway.
Why bother?
Same things for our boats.
Indeed, one of the most valuable diagrams I drew was the wiring behind my nav station instrument panel, which never appeared in any OEM diagrams anyway.
And even if your goal is to eventually replicate what was supposed to be there, please know that electrical systems of that vintage have been superseded by newer and better bare bones designs.
Good luck.
