Taking a flier here. My car has been down and sitting at my local dealership for over 4 months. It's a brick. Ignition failure. It was throwing codes regarding the AFM system, specifically the exhaust valve actuators. It was determined that there's a short in the Engine Wiring Harness. Apparently this is the hardest to find part for C8's in history. While I was waiting a VERY good friend who does OEM work for GM was able to get me in touch with someone who was able to get their hands on a harness for me, which I gladly paid for. Here's the rub: the GM part number I bought, 85640858, is different from the one my dealership has on order, 85635878. From what I was able to find from public information, 5878 is a 2nd DES and 0858 is 1st DES. It's not clear if the part I have in-hand supersedes the one on order. From what I can tell just about every 2nd DES involves improvements in insulation/shielding and fitment issues (additional stress relief etc...) and not the pin outs of the connectors themselves. IF there were any changes in the pin out that would indicate additional sensors were added but there would also need to have been a TSB explaining why. There aren't any. So, what I'm trying to figure out-and I'm willing to all of the legwork that I can-is where can I search to find out the differences between the 2 parts. (They both fit my VIN according to my dealerships parts department ordering system, which I had eyes-on). My dealership understandably doesn't want to install a part that might not work, so again I'm in a permanent holding pattern. Thanks in advance for any insight!

 

At some specific date, at the assembly plant, the new wiring harness will show up on the assembly line.
The assembly plant probably will not know why this wiring was changed, or what was improved.
They typically only know that a new wiring part number is starting on the assembly line today.

The dealer will be the last one to know about anything that changed, and they will know very little since they only deal with repairs, not new car assembly.
That is the nature of wiring harness revisions, and they are often very complicated and are phased in with revisions of other electrical components.
So you are really a pioneer here, trying to find out what changed, what improved, and why, and will that change to the wiring harness work on your car. ???
All of the information you need is in some engineers head, or GM engineering wiring harness documentation, not public knowledge, not TSB. It would probably be easier to find Bin Laden.