It depends on how the system is configured.
When wiring speakers, I use the following conventions, positive side first:
red/black
copper / silver
solid / striped
If you're not sure, run a test tone through the speakers. If it sounds awful, then reverse the wiring.
You will not damage the speakers by wiring them incorrectly, as long as you are wiring them to the stereo. If you do something daft like try to put them into a wall outlet you'll probably fry them. Not that you'd care since you'd be dead and your house would have burnt to the ground.
According to color code, it is the speaker negative wire.
The car stereo wiring colors for a 1988 Cadillac El Dorado with Bose are gray for right front speaker positive. Light green for right front speaker negative. Dark green for audio rear speaker positive and brown for rear speaker negative.
1998 Toyota Tacoma Stereo Wiring InformationRadio Battery Constant 12v+ Wire: Blue/YellowRadio Accessory Switched 12v+ Wire: GrayRadio Ground Wire: BrownRadio Illumination Wire: GreenLeft Front Speaker Positive Wire (+): PinkLeft Front Speaker Negative Wire (-): PurpleRight Front Speaker Positive Wire (+): GreenRight Front Speaker Negative Wire (-): BlueLeft Rear Speaker Positive Wire (+): BlackLeft Rear Speaker Negative Wire (-): YellowRight Rear Speaker Positive Wire (+): RedRight Rear Speaker Negative Wire (-): White
1996 Nissan Altima Car Stereo Radio Wiring DiagramCar Radio Constant 12v+ Wire: PinkCar Radio Switched 12v+ Wire: BlueCar Radio Ground Wire: ChassisCar Radio Illumination Wire: Red/BlueCar Stereo Dimmer Wire: Red/YellowCar Stereo Antenna Trigger: Green/WhiteCar Stereo Amp Trigger Wire: N/ACar Stereo Amplifier Location: N/AFront Speakers Size: N/AFront Speakers Location: N/ALeft Front Speaker Positive Wire (+): Blue/WhiteLeft Front Speaker Negative Wire (-): Blue/YellowRight Front Speaker Positive Wire (+): BrownRight Front Speaker Negative Wire (-): Brown/WhiteRear Speakers Size: N/ARear Speakers Location: N/ALeft Rear Speaker Positive Wire (+): RedLeft Rear Speaker Negative Wire (-): GreenRight Rear Speaker Positive Wire (+): BlueRight Rear Speaker Negative Wire (-): Pink
Car Radio Constant 12v+ Wire: Orange Car Radio Switched 12v+ Wire: Yellow Car Radio Ground Wire: Black Car Radio Illumination Wire: Gray Car Stereo Dimmer Wire: Brown Car Stereo Antenna Trigger: Pink 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 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
2000 olsmobile aleroCar Radio Stereo Audio Wiring DiagramCar Radio Battery Constant 12v+ Wire: OrangeCar Radio Accessory Switched 12v+ Wire: YellowCar Radio Ground Wire: Black/WhiteCar Radio Illumination Wire: Gray/BlackCar Stereo Dimmer Wire: Brown/WhiteCar Stereo Antenna Trigger: Dark GreenCar Stereo Amp Trigger Wire: N/ACar Stereo Amplifier Location: N/ACar Audio Front Speakers Size: N/ACar Audio Front Speakers Location: N/ALeft Front Speaker Positive Wire (+): TanLeft Front Speaker Negative Wire (-): GrayRight Front Speaker Positive Wire (+): Light GreenRight Front Speaker Negative Wire (-): Dark GreenCar Audio Rear Speakers Size: N/ACar Audio Rear Speakers Location: N/ALeft Rear Speaker Positive Wire (+): BrownLeft Rear Speaker Negative Wire (-): YellowRight Rear Speaker Positive Wire (+): Dark BlueRight Rear Speaker Negative Wire (-): Light Blue
1998 Buick Skylark Car Radio Stereo Wiring Diagram Car Radio Battery Constant 12v+ Wire: OrangeCar Radio Accessory Switched 12v+ Wire: YellowCar Radio Ground Wire: BlackCar Radio Illumination Wire: GrayCar Stereo Dimmer Wire: BrownCar Stereo Antenna Trigger: N/ACar Stereo Amp Trigger Wire: N/ACar Stereo Amplifier Location: N/ACar Audio Front Speakers Size: N/ACar Audio Front Speakers Location: N/ALeft Front Speaker Positive Wire (+): TanLeft Front Speaker Negative Wire (-): GrayRight Front Speaker Positive Wire (+): Light GreenRight Front Speaker Negative Wire (-): Dark GreenCar Audio Rear Speakers Size: N/ACar Audio Rear Speakers Location: N/ALeft Rear Speaker Positive Wire (+): BrownLeft Rear Speaker Negative Wire (-): YellowRight Rear Speaker Positive Wire (+): Dark BlueRight Rear Speaker Negative Wire (-): Light Blue
2002 Chevrolet Avalanche Car Stereo Wiring DiagramCar Radio Battery Constant 12v+ Wire: OrangeCar Radio Accessory Switched 12v+ Wire: YellowCar Radio Ground Wire: BlackCar Radio Illumination Wire: BrownCar Stereo Dimmer Wire: GrayCar Stereo Antenna Trigger Wire: N/ACar Stereo Amp Trigger Wire: N/ACar Stereo Amplifier Location: N/ACar Audio Front Speakers Size: N/ACar Audio Front Speakers Location: N/ALeft Front Speaker Positive Wire (+): TanLeft Front Speaker Negative Wire (-): GrayRight Front Speaker Positive Wire (+): Light GreenRight Front Speaker Negative Wire (-): Dark GreenCar Audio Rear Speakers Size: N/ACar Audio Rear Speakers Location: N/ALeft Rear Speaker Positive Wire (+): BrownLeft Rear Speaker Negative Wire (-): YellowRight Rear Speaker Positive Wire (+): Dark BlueRight Rear Speaker Negative Wire (-): Light Blue
2002 Toyota Corolla Car Radio Stereo Audio Wiring DiagramCar Radio Constant 12v+ Wire: Blue/YellowCar Radio Switched 12v+ Wire: GrayCar Radio Ground Wire: BrownCar Radio Illumination Wire: GreenCar Stereo Dimmer Wire: N/ACar Stereo Antenna Trigger: N/ACar Stereo Amp Trigger Wire: N/ACar Stereo Amplifier Location: N/ACar Audio Front Speakers Size: N/ACar Audio Front Speakers Location: N/ALeft Front Speaker Positive Wire (+): PinkLeft Front Speaker Negative Wire (-): VioletRight Front Speaker Positive Wire (+): Light GreenRight Front Speaker Negative Wire (-): BlueCar Audio Rear Speakers Size: N/ACar Audio Rear Speakers Location: N/ALeft Rear Speaker Positive Wire (+): BlackLeft Rear Speaker Negative Wire (-): YellowRight Rear Speaker Positive Wire (+): RedRight Rear Speaker Negative Wire (-): White
as describe by modifiedlife.com 2001 Nissan Sentra Car Stereo Radio Wiring Diagram Car Radio Constant 12v+ Wire: PinkCar Radio Switched 12v+ Wire: BlueCar Radio Ground Wire: GrayCar Radio Illumination Wire: Red/GreenCar Stereo Dimmer Wire: Red/YellowCar Stereo Antenna Trigger: N/ACar Stereo Amp Trigger Wire: N/ACar Stereo Amplifier Location: N/AFront Speakers Size: N/AFront Speakers Location: N/ALeft Front Speaker Positive Wire (+): PinkLeft Front Speaker Negative Wire (-): BlueRight Front Speaker Positive Wire (+): WhiteRight Front Speaker Negative Wire (-): BlackRear Speakers Size: N/ARear Speakers Location: N/ALeft Rear Speaker Positive Wire (+): Light GreenLeft Rear Speaker Negative Wire (-): VioletRight Rear Speaker Positive Wire (+): BlueRight Rear Speaker Negative Wire (-): Pink
Car Radio Battery Constant 12v+ Wire: OrangeCar Radio Accessory Switched 12v+ Wire: YellowCar Radio Ground Wire: BlackCar Radio Illumination Wire: GrayCar Stereo Dimmer Wire: BrownCar Stereo Antenna Trigger Wire: PinkCar Stereo Amp Trigger Wire: N/ACar Stereo Amplifier Location: N/ACar Audio Front Speakers Size: 4″ x 6″ SpeakersCar Audio Front Speakers Location: DoorsLeft Front Speaker Positive Wire (+): TanLeft Front Speaker Negative Wire (-): GrayRight Front Speaker Positive Wire (+): Light GreenRight Front Speaker Negative Wire (-): Dark GreenCar Audio Rear Speakers Size: 6″ x 9″ SpeakersCar Audio Rear Speakers Location: Rear DeckLeft Rear Speaker Positive Wire (+): BrownLeft Rear Speaker Negative Wire (-): YellowRight Rear Speaker Positive Wire (+): Dark BlueRight Rear Speaker Negative Wire (-): Light Blue
An electromagnet drives the stereo speaker to provide the sound.