I would like to suggest an OEM Car Audio Wiring Forum Where you can look up the OEM stereo and get the Wiring Diagram for it.
Sometimes People want to keep their After Market stereo when they sell their car and they get an OEM stereo to put back into the car. If the wires do not match up then you can wire the OEM radio into the existing harness if you Know which wire goes where.
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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
Maybe I am not understanding you but if you put the factory head unit back in the origanal vehicle it will plug right in. But maybe you are talking about the hacks that don't use wiring kits. 
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The Clear Bra Guy
KCs premiere paint protection guru
Here's something along the lines of what you're looking for in the downloads section... https://www.the12volt.com/downloads/downloads.asp?catid=49
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the12volt • Support the12volt.comYes The HACKS as you call them. I have had people bring me a Stereo fom say a Ford and want it installed into a Chevy. Well it fits but the wires don't match up All are hacked and well I just do the best that I can by looking up the IC's in the radio and researching a pinout for the IC following the traces on the circuit board and finding a solution from tech-data only.
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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
I think that your suggestion is great. Right now I have a cheap '89 Sentra, but I bought a 2000? factory Saturn stereo to put into it for temporary toons (besides the am/fm) and the wires are different from all the diagrams I've looked at. I posted a question about the wiring.... any suggestions? :)
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shredntiresNmymachI
Something Like this website!
https://carstereohelp.net/wireharness_Delco1.htm-------------
John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
like this website!
https://carstereohelp.net/wireharness_Delco1.htm
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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
Thanks for the pic of the stereo/harness, but it's totally different. I ended up running down to the Saturn dealership and they still couldn't find the right connector or wiring (colors). They said my OEM stereo was from a '96-'99 Saturn, but the harness pins were set up differently & also the colors didn't match. I was told my best bet would be to run down to the local GMC dealer to try my luck there. What I thought was a great buy is turning out to be a real headache! :) If I just decided to try matching it up wire by wire, starting with the ground & (what I think is the) switched, then constant & so on..... would it be too much of a risk of frying something? The stereo doesn't have an inline fuse... should I throw one on? I may just try the dealership or wait until I find a different stereo, one with easily to obtain wiring info.... :)
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shredntiresNmymachI
........actually, the #2 wiring diagram plug looks like mine, although mine doesn't have the plug built into the stereo. As for the wiring colors, mine are totally different, although the ground wire is in the same place.
PINS:
(1) brown, (2) lt green, (3) gray, (4) blue
(5) black, (6) yellow, (7) violet, (8) NOT USED, (9) red, (10) orange
(17) brown, (18) lt green, (19) gray, (20) blue
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shredntiresNmymachI