OK so I did some work with the DMM this weekend.

The yellow wire is constant positive. Guessing PO spliced this in somewhere to give constant power for aftermarket radio memory.

Speaker wires and antenna wires are self explanatory.

Then I have that double wire bundle. The black wire is positive power with ignition on. The green/red striped wire has 0 resistance to ground, so this appears to be my ground wire.

Looking at wire diagram, the colors are all wrong. That bundle should be red for positive and brown for ground. My car is a Euro model, would that be different? The wires look factory, but maybe it was done by PO?

More important: where is my blue wire for the power antenna? Any ideas where I can find it? I dug around in there and no sign of it. I'm not sure I'm super ambitious to install a radio if I can't get the antenna to go up...

Thanks in advance.