Finally got around to adding an aux input to my factory radio. I tried the Bluetooth board I bought, but kept having ground noise issues, so I just used an aux cable instead.
I soldered the left and right wires of the aux cable to the L and R #2 inputs(AM Radio) on the preamp chip and ground to ground. This way I still get to use the chips equalizer and balance settings. I desoldered the AM tuner connections so that I won't get any buzzing. So now if I want AUX, I set the radio to AM. FM Radio still works great.
Here's some pics and the data sheet of the chip in my radio. Note - The left and right are shown here as being soldered to the #3 inputs. I switched these later to the #2 inputs, overriding the AM signal.
My phone puts out too much voltage, so I'll have to add a 100k trimmer pot so I can tone it down, but my battery powered 3.5mm Bluetooth receiver works great.