Need pics or diagram of how wiring runs under dash

Try Faxon (repop factory manual seller). I got an assembly book for 70-71 a-bodies and they have diagrams for all of that...
I'll second that. The assembly manual is very helpful and shows a lot of things the service manual doesn't. The Faxon watermarks right on the engineering illustrations drive me nuts, but it's a go to book.
I just checked. There don't seem to be any reprints for later than 1972 model year.