yeah, unfortunately, the wiring diagram is gonna be your best bet.
google for the radio install, it should break down a list of what wires are actually supposed to go to the radio, that will help sort out what is for speakers and what is for the power antenna and crap like that.
From there, break out the wiring diagram and start labeling each wire as soon as you find its color. The wiring diagram will have the plugs that go to the different places, like the cig lighter and head unit and all that.
it is tedious but thats really the only way to figure out what was cut from where and attempt to get it back to stock.

I never will get why radio install places have to jack stuff up so bad. all of the info they need is on the internet to do a clean, simple job. Don't get why they waste their time guessing and hacking up the wires the way they do.



The other option is to always just yank the dash out and replace that whole harness.
I'm sure someone that always has parts cars floating around, like z28ricer, should have the harness you would need.