I've done this a couple of times on both the ribbon style and individual wire style. Here is what I did on the ribbon style:
I cut the ribbon to separate the individual circuits and tapped off the right and left channels.
Then plugged it in.
I found a place to solder a wire into the right and left circuits.
I soldered into the right and left circuits on the bottom side of the radio board.
I installed the AUX jack just to the right of the EQ. I had to make the plastic face thinner with a dremel.
No modification of the circuitboard was required to get the AUX jack to fit in this location.
The finished product.
As a note, on both of the radios that I did the mod on the display bulb went out within a month or so. In the future I will replace the bulb during the mod. The bulb is available from GM. The part number is 16208443. Last time I checked it was cheaper to get it from your local dealer than from an online store (amazon, rockauto, etc).
