Assuming you have a non M, follow this link to access the back wall...
http://www.zpost.com/forums/showthread.php?t=497843
Look at the second last pic and you will see a silver box on the wall beyond the 2x4 in the pic. That's the TCU which has the bluetooth code on the inside of the box.
If you do not have the sheet with the code and cannot see the box from behind the speaker, then this link has the best method to make 100% certain you do not have the box. If it IS there, the other method is to remove the screws on the bracket for this box and get a little mirror behind it to read the code...that's what I did.