If someone can post some clear, close-up pictures of connector "i26" (the 15 [correction -- 16] pin connector that you're adding the aux wires to), I may be able to get the correct pins to snap into the plug. Perhaps even the shells to make a plug-and-play insert harness.
I need to see the pins side of both plug and socket. The top, bottom, and either side of the car's harness plug would be helpful, too.
Here's the back (wires side), from kmartin's PDF install document:
Pin 13 (where the braided shield wire goes in that picture) is the aux enable line. That pin should be tied to chassis ground (and not used for the aux audio ground).
Pin 14 (empty, next to the white wire) is the aux audio ground.
You can also use a cable with 3.5mm male to male, or male to female rather than one with RCAs -- might be slightly less expensive, and you'll only have one cable with one braided shield you'll need to strip.