Maybe it was for a cassette changer? I don't even know if the 1989 model had CD players as an option.
At any rate, I installed the radio and left the mystery wire alone, and the radio works great. It's running on 2 channels but it sounds good, I'm not an audiophile... I might run new wires to the speakers, but that'll be more for an excuse to tinker with it
Here are some more pictures for anyone else who might need them:
These are the wire harnesses, on the left is the car-specific harness from Crutchfield, on the right is the player-specific harness that came with the Sony player.
Roughly from top to bottom, the wires are:
Yellow - 12 volts constant power
Orange - illumination - not used by the Sony player, I just taped it off
Red - 12 volts switched power
Black - Ground
These 4 connect to the plug on the left in my first picture in this thread.
Blue - power antenna control (connect to middle wire in my first picture in this thread - more on this one later)
Blue/white - amp control (unused for me)
Gray + Gray/black - front right speaker - connected to "right" (black) speaker connector in car
Violet + Violet/black - back right speaker (unused for me)
Green + Green/black - back left speaker (unused for me)
White + White/black - front left speaker - connected to "left" (white) speaker connector in car
The power antenna wire from the Sony harness gave me a bit of trouble. It looks like this:
which doesn't really fit with the middle wire from the first picture. I had to cut it and crimp a male connector instead:
One last thing - the panel underneath the radio has two plastic hooks which go up into two holes in the bottom of the factory-installed player. The new player doesn't have those holes, meaning the panel can't slide all the way up, meaning I couldn't put the ashtray back in. I saw no other option but to amputate. This is the back of the panel after the operation:
And finally, the finished product:
It's not much to look at, but it probably fits better with the car interior than a flashier Alpine player.