[NoQuarter]

I have not been able to find this in the wire diagrams. In right side dutch door (where the license plate frame is), is this unused factory harness. In the pic you can see the unused speaker connection on the right, a larger connection that seems to be power based on the size of the wires, and 2 bare spade terminals.

 

[Rustbelt_mVan]

There were two different levels of rear speaker available depending on trim line. The "Premium Radio" shows a connector for a "subwoofer" in the RR door

 

[NoQuarter]

That power wire is interesting though. The subwoofer would have had an amp mounted in the driver side panel about midway back so the subwoofer would not have been powered. Unless those thicker gauge wires are not power and are thicker speaker wires for the subwoofer. That leaves the bare spade connectors...

 

[AstroWill]

The only things that were in the rear doors were speakers, door jam switch, license plate light, and lock actuator, then you have the liftgate stuff as well. Maybe the wiring was always installed regardless of which doors were ordered, not sure.