GMC 2000 Sierra - Manual (Page 186)

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CD Adapter Kits
It is possible to use a portable CD player adapter kit
with your cassette tape player after activating the bypass
feature on your tape player.
To activate the bypass feature, use the following steps:
1. Turn the ignition to RUN or ACCESSORY.
2. Turn the radio off.
3. Press and hold the TAPE AUX button for five
seconds. The tape symbol on the display will flash
for two seconds, indicating the feature is active.
4. Insert the adapter into the cassette slot. It will power
up the radio and begin playing.
This override routine will remain active until EJECT
is pressed.
AM-FM Stereo with Compact Disc Player
and Automatic Tone Control (If Equipped)
Playing the Radio
PWR - VOL: Press this knob to turn the system on and
off. To increase volume, turn this knob clockwise. Turn
it counterclockwise to decrease volume. The knob is
capable of rotating continuously.
RECALL: Display the time with the ignition off by
pressing this button. When the radio is playing, press
this button to recall the station frequency.