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Hi there,
Welcome to the forum, I did some searching and found this.

does this help?

2005 Buick Lacrosse Car Radio Stereo Wiring Diagram

Car Radio Battery Constant 12v+ Wire: Orange
Car Radio Accessory Switched 12v+ Wire: The radio harness does not provide a switched power source. Run a wire to the fusebox for switched power.
Car Radio Ground Wire: Black/White
Car Radio Illumination Wire: N/A
Car Stereo Dimmer Wire: Gray
Car Stereo Antenna Trigger: Purple
Car Stereo Amp Trigger Wire: Same as Power Antenna Trigger
Car Stereo Amplifier Location: Located on the rear deck lid.
Car Audio Front Speakers Size: N/A
Car Audio Front Speakers Location: N/A
Left Front Speaker Positive Wire (+): Tan
Left Front Speaker Negative Wire (-): Gray
Right Front Speaker Positive Wire (+): Light Green
Right Front Speaker Negative Wire (-): Dark Green
Car Audio Rear Speakers Size: N/A
Car Audio Rear Speakers Location: N/A
Left Rear Speaker Positive Wire (+): Brown
Left Rear Speaker Negative Wire (-): Yellow
Right Rear Speaker Positive Wire (+): Dark Blue
Right Rear Speaker Negative Wire (-): Light Blue

I'm putting a stereo in for my cousin and after pulling out the stock stereo, I couldn't find the wiring diagram for the stock wire harness.
I found a few sites that showed most of the wires but didn't include everything.
Any information is useful.



Thanks.