GMC 2003 GMT560 T-Series Owner's Manual page 116

The cut tape detection feature of your cassette tape
player may identify the cleaning cassette as a damaged
tape. To prevent the cleaning cassette from being
ejected, use the following steps:
1. Turn the ignition on.
2. Turn the radio off.
3. Press and hold the TAPE button for two seconds
until the radio produces one beep. CAS will flash
on the display for five seconds.
4. Insert the scrubbing action cleaning cassette.
5. Eject the cleaning cassette after the manufacturer's
recommended cleaning time.
After the cleaning cassette is ejected, the cut tape
detection feature will be active again.
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You may also choose a non-scrubbing action, wet-type
cleaner which uses a cassette with a fabric belt to
clean the tape head. This type of cleaning cassette will
not eject on its own. A non-scrubbing action cleaner
may not clean as thoroughly as the scrubbing type
cleaner. The use of a non-scrubbing action, dry-type
cleaning cassette is not recommended.
After you clean the player, press and hold the eject
button for three seconds to reset the CLN indicator. The
radio will display --- to show the indicator was reset.
Cassettes are subject to wear and the sound quality may
degrade over time. Always make sure the cassette
tape is in good condition before you have your
tape player serviced.