I don't know what year CTS you are driving or if it makes a difference, but this is from the 2011 CTS owner's manual (please refer to your own manual for further instructions):
Connecting an iPod
To connect the iPod, connect one
end of the special iPod connection
cable to the iPod’s dock connector.
Connect the other end to both the
USB port and the auxiliary input jack
located in the center console. The
USB port and the auxiliary input jack
are located in the center console
bin. If the vehicle is on and the USB
connection works, a GM logo may
appear on the iPod. The iPod's
music information will be shown on
the radio’s display and the music
begins playing through the vehicle's
audio system.
The iPod's battery charges while
it is connected to the vehicle
and if the ignition is turned to
ACC/ACCESSORY or ON/RUN.
It can also be left connected to the
vehicle after the vehicle is turned
off. With the vehicle turned off, the
iPod will automatically be powered
off and will not charge or draw
power from the vehicle's battery.
A standard iPod USB cable, like
the one that came with the iPod,
cannot be used to connect an iPod
to the vehicle. The special iPod
connection cable that came
equipped with the vehicle or that
was made available from your
dealer must be used to command
and control an iPod.
Use a standard 3.5 mm (1/8 in)
stereo cable to connect an older
iPod model that is not supported,
or if the special iPod connection
cable is missing. Command and
control of the iPod using the
infotainment control buttons and
knobs is not supported when only a
3.5mm (1/8 in) stereo cable is used.
Commanding and Controlling an iPod
An iPod™ can be controlled by
using the radio buttons and knobs;
the song information shows on the
infotainment system's display.
To connect and control the iPod
using the radio controls, use the
special iPod connection cable that
came as standard equipment with
the vehicle as part of the USB
option. The special iPod connection
cable can also be purchased or
made available, from your dealer.
See your dealer for more
information.
Use the Menu/Select knob to bring
up the iPod Menu and select Songs,
Artists, Albums, Playlists and Audio
books to play from the iPod.
Disconnecting an iPod
To properly dismount the iPod
USB device before disconnecting
from the vehicle, press the softkey
under the EJECT tab on the
infotainment display screen during
iPod playback.