The wiring harness. To the stereo has been cut. I need to install oem stereo and don't know where to start to find which wire hold the ground or power.

1998 Chevrolet Malibu Car Stereo Wiring Diagram

Car Radio Battery Constant 12v+ Wire: Orange
Car Radio Accessory Switched 12v+ Wire: Yellow
Car Radio Ground Wire: Black
Car Radio Illumination Wire: Gray
Car Stereo Dimmer Wire: Brown
Car Stereo Antenna Trigger Wire: Pink
Car Stereo Amp Trigger Wire: N/A
Car Stereo Amplifier Location: N/A
Car Audio Front Speakers Size: 4? x 6?
Car Audio Front Speakers Location: Doors
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: 6? x 9?
Car Audio Rear Speakers Location: Rear Deck
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

Hope this helps (remember to rate this).

Since you are planning to install an OEM headunit / radio, you can replace the factory with an aftermarket plug, see included link:

GM harness into radio

The factory colors are as follows:

Plug C2:
Constant 12V = Orange
Ground = Black / White
Ignition 12V = Yellow
RF Speaker (+) = Light Green
RF Speaker (-) = Dark Green
LF Speaker (+) = Tan
LF Speaker (-) = Gray
Dimmer = Gray
Illumination = Brown

Plug C1:
RR Speaker (+) = Dark Blue
RR Speaker (-) = Light Blue
LR Speaker (+) = Brown
LR Speaker (-) = Yellow
VSS Input = Dark Green / White (for use with speed controlled volume)

Plug C3: (connects to external cassette player)
Constant 12V = Orange
Ground = Black / White
Remote Turn-on = Pink
Dimmer = Gray
Audio Common = Light Blue
Right Aux. Signal = Light Green
Left Aux. Signal = Orange / Black
E & C Buss = Dark Green (sorry cannot describe function of this wire)