Update:
I laboriously traced everything out and now have a handle on what all the spaghetti is.

There is a large harness with many wires which snakes forward to parts unknown under the dash.

But I also traced all the needed power, ground and antenna trigger wires coming in from the harness separately.
All these wires go into the connectors and then into the mystery harness and then back out again to the HU.

The rest are the speaker wires. The fronts go more or less directly from the HU through a central connector to the speaker wires. But the rears go from the HU into the mystery harness and apparently back out again but with all different colors from what they go in as.
I can only conclude that there is a separate amp hidden in the dash somewhere that drives the rear speakers. I'll probably have to pull the central console to find it.