Well, I have done some hunting and found the following:

1- The item cut off was to the R taillight harness. I had to splice in a Deutsch connector to connect the light harness to the bare wires. This is the Green circle
2- The Yellow circle appears to be the backup camera connector. The backup camera harness is the only one with 8 pins as well, and the structure appears to be the same. I have ordered a harness to ensure this is correct, but I'm 95% sure it is.
3- The red circle appears to be for the park assist harness. It is the only GM harness remotely located in this space with a 16-pin configuration. While not all the pins are used, I cannot find any other harness in the GM stores that runs a 16-pin configuration. I have ordered this harness (and a bumper/sensors/etc) and will report back, but again I'm 95% certain this is correct.

If anyone runs into an issue with a service-bed truck and needs this info in the future, here you go!

 

Also, harness plug directly to the left with the 4 exposed wires and red tab was to the Left (drivers) tail light harness. I still have not confirmed what the 'empty' harness is on the very left. Has a purple/black, white/purple, and black wire, but w/o any wiring diagram, I'm unsure of where it goes. On my truck, the receptacle plug is blocked, so maybe an 'option' that they plugged off on my truck? Power Tailgate??? Dunno.