Use the proper interface

http://www.pac-audio.com/databusVehicles/GMLANvehiclesdetail.asp?mmSearch=20

and, if it has onstar, you'll need a different version.

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Alpine CDA-9887, TUA-T500HD, CHA-S634, CHA-1204 & KCA-410C
Pioneer TS-A6981, 6 x GM-5100T
Kicker 2 x CVR15,
MBQ RCE-216
PG MX2
Soundgate LanStar1
PAC SWI-ALP