If you are trying to add an AUX jack to override CD sound, read on...
 

Hello.
My radio does not have RCA jacks or an AUX connection or even Bluetooth but I wanted to play music from my phone via the radio without using an FM transmitter or one of those cassette adapters.

There are quite a few instructions out there on how to do this, from removing the radio from the dash, opening it up, removing the CD player and hooking up a 5 pin AUX jack to the left/right/gnd wires going to the CD player. This will allow you to continue using the CD player to play CDs but when you want to play music from your phone/mp3 player, it will override the sound coming from your cd's and use the music from your phone instead. I won't expand on that part.

However, most examples point to locating a 14/16 pin connection on the CD player and using the first and third pin to get your sound (with pin #2 being ground).
However, my radio had two sets of connections going to the CD player. One had 14 pins and the other had 6 pins. From my research, the sound (left/right channels) come out of that 6 pin connection, not the 14 pin one.
The black wire in the 6 pin connection is one of the channels (consider that one pin #1) and the third pin is the other channel. You can use the metal case that holds the equipment as your ground.
The rest of the instructions apply in regards to the actual connection to the AUX jack.
Thought I'd share this if someone was interested (like I am) in adding an AUX jack to override the sound from the CD player when you wish to play music from your phone as an example.

Funny you share this. I'm researching the same topic. Except my issue is that I don't want to inject the audio via the CD player (because then you have to keep a CD in the player running all the time, pointless wear and tear on the unit).

Ideally, I'd like to know how to interrupt the audio going to the pre-amp (then it would not matter if I was using CD or radio). I have been unable to find data on this. I found a computer that well refurb the radio and add the aux jack for $150 and they claim they inject the audio at the FM modulator to interrupt it with what ever you want (for more $$ they offer a bluetooth add-on).

Another common one is for people with XM to interrupt the XM stream with whatever you want. I happen to have a 04 Grand Am with factory XM and I've done some testing to see how I might be able to add XM to the Monte (only to abandon the XM to add an AUX jack). I was able to interface the XM receiver from my Grand Am to my Monte (for testing, I jumped the power/ground/data in through the diagnostic port under the dash). Doing that now gave the radio the XM option (since the receiver was not programmed for the car, it said LOCKED but I believe it still allows audio, but since I did not want to rip the Monte dash apart to further test, that's where I left it).

If you can source the XM parts cheap, I believe it would be fairly easy to add to the Monte (especially if you did not care about the antenna). I found that on the back of the radio, there is a pin on the 12 pin connector that passes a data bus connection. To add XM you would need to fabricate a harness to grab data from the radio, set up the audio how you want (either AUX only or use a nice jack that let's you inject AUX when you want and XM rest of the time) and then find where to grab power and ground.

I would rather NOT go the XM route, rather find a cleaner way to go about it.