I have my OEM head unit out at the moment, so I can easily look at my connector and tell you which wire is where. I have an 8-speaker system (IS200 Sport) too.
Are you looking at this diagram ? I can see 14 wires (not 13) on that diagram. The 15th wire is the green one for illumination in pin 2 on this diagram.
So that sounds like the 15 that you're after. Let me know if you still want me to look at my connector in my dash.
Edit - looking again, I take that back, looks like 12 wires on the first diagram with the 13th on that illumination diagram into the main connector, those other wires are on a different connector. My offer still stands of course.