1985 745t

This Christmas I got a Rio Karma DAP, and I've been using it in my car with a standard tape deck converter. Previously I had been using the same set-up for a cd player, so I'm pretty comfortable with wires and the set-up. However, given that I've added extra cigarrette lighter ports and other stuff for convenience to my brick, I've decided I'd also like to incorporate an aux input if possible for connecting to the Karma and any other secondary devices in the cabin.

I'm thinking something really simple here, like hacking a Y wire and splicing the leads directly into the stereo's existing wiring. However, the stereo is a bit of a jumble to me, and frankly I have no idea what the existing system is capabale of, what the wiring is (no labels), or how to approach this project.

My stereo is the OEM version with tape deck for the 1985 year (I've attached a picture at bottom). It has a volume dial on the left, a tuner dial on the right, screen mid left, 6 pre-sets to the right of the screen, two AM and two FM pre-set groups, Bass/Treble levers below the volume, etc. I'd like to know the following:

1) Was the OEM stereo made to handle a secondary device like a CD changer?

2) If so, do any of the wires on the back of this thing server as inputs? Is there a "custom interface cable" like there is with later models as described in the FAQ?

3) Has anyojne done this before and is there a resource or guide available on-line somewhere?

I've got to assume I'm not the first person to tackle this project, and that with the plethora of DIY folks on this board someone has done something pretty elegant already. I couldn't find any posts laying it out for me, so any help or insight is greatly appreciated.

Thanks,
Rick