Are you sure there is one? i know they play mp3's but isn't that just from an CD?

you can get things to plug into your ipod like an itrip that plays it through the radio.

I called GM service and I gave my VIN # and they said I should have one.  The customer service rep that I had on the phone did not sound confident when they told me I had one, I had to be put on hold.  I will take a look for the itrip sounds like it would be a better option then call customer service back.  Thanks