K. Where to find the factory CD changer?

CD changer: 

• It is mostly in the trunk, sometimes in the glove box, in rare cases in the center console.
• If the CD changer connection was too far in the car, then you can use the optical extension kit (FOA1PO1) for the installation, which makes it possible to place the hardware of the Gateway to the front part of the car, into the glove box for example.

As an example here is an old Audi 6CD changer.