I need to know what the various colored wires connect to since I needed to remove the old amp and install an after market stereo.

read (you meant main harness to radio sub harness.)
and car may have 5 or more harness for 5 or more very complex radio head end options new,so only the real service manual covers all that for sure,or if lucky 2nd fiddle here.

https://eautorepair.net/Marketing/Default.asp

Question edited into English :>D

2004 Subaru Impreza Car Radio Wiring Guide - MODIFIEDLIFE

https://www.modifiedlife.com > 2004-subaru-impreza-c...
Feel free to use any car stereo wiring diagram or factory radio wire colors that are listed on Modified Life but keep in mind that all information here is ...

Radio Wiring Diagram for a 2004 Subaru Impreza Wagon - Fixya

https://www.fixya.com > Forum > Car Audio & Video
Nov 15, 2019 - ... 2007 Subaru Impreza Car Stereo Wiring Diagram. Car Radio Battery Constant 12v+ Wire: Blue/Red Car Radio Accessory Switched 12v+ Wire: Yellow/Green
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Subaru Impreza WRX 1993-2007 Car Stereo pinout

https://pinoutguide.com > ... > Subaru Impreza pinouts
Dec 9, 2019 - Pin #. Function. WRX Factory Radio Connector Wire Color ( wires go to WRX's Speakers, etc.) Subaru After-Market Harness Wire Color

How To Subaru WRX Stereo Wiring Diagram - My Pro Street

https://my.prostreetonline.com > 2016/02/15 > how-to-s...
Feb 15, 2016 - Use our WRX Stereo Wiring schematic to complete your aftermarket stereo install into your Subaru Impreza WRX or STI.

Here is a link to the wiring diagrams. http://bbbind.com/free_tsb.html

If it's only 4 wires, Red & black are typically power. red +12volts & black is ground. Gray would be one channel say right side and green would be left (example) you would ground the negative speaker lead on both sides. That is probably an old unit and make sure your car is negative ground. Some Ford & Nissan are positive grounded for some circuits which you may need to reverse the power connections. Make sure there is an in line fuse on the power wire just in case.

what you need is to go on line " crutchfield " .com , they have wire diagrams for all vehicles, and aftermarket stereos , you need to know how to use your existing stereo wires to connect your aftermarket stereo wires to these wires ! crutchfield can show you how to decifer the color codes so you can connect these wires ! its very easy, just buy somthing from them , like the connectors you'll need to do this, get it. good luck

Are you the original owner ? Did it have an after market radio installed ? If yes, it might of had wiring connected wrong from there.

You'll need to go to support site for manufacturer of stereo and check for a download for owners manual which will give wiring diagram for stereo. Have your model number ready. Then go to (http://www.the12volt.com/) and they have wiring info for your vehicle.

sounds the like the vehicle had an oem amplifier that was removed, most after market stereo installers will trace the speakers wires individually by continuity to by pass the factory amplifier and wire the new head unit to the vehicles speakers, if the harness has been spliced or factory amp removed, you will need to check if the amplifier still exists (usually under passenger rear seat sometimes located in trunk area) and see if the harness has been bypassed. if so you will have to reconnect the harness by color code in order to use the factory head unit

1994 Volvo 940 Car Audio Wire Diagram
Car Radio Battery Constant 12v+ Wire: Yellow
Car Radio Accessory Switched 12v+ Wire: Red
Car Radio Ground Wire: Black
Car Radio Illumination Wire: Light Orange
Car Stereo Dimmer Wire: N/A
Car Stereo Antenna Trigger Wire: N/A
Car Stereo Amp Trigger Wire: N/A
Car Stereo Amplifier Location: N/A
Car Audio Front Speakers Size: 5 1/4′
Car Audio Front Speakers Location: Doors
Left Front Speaker Positive Wire (+): Blue/Yellow
Left Front Speaker Negative Wire (-): White
Right Front Speaker Positive Wire (+): Gray/White
Right Front Speaker Negative Wire (-): Gray
Car Audio Rear Speakers Size: 6′ x 9′
Car Audio Rear Speakers Location: Rear Deck
Left Rear Speaker Positive Wire (+): Yellow/Brown
Left Rear Speaker Negative Wire (-): Yellow/Green
Right Rear Speaker Positive Wire (+): Green/Brown
Right Rear Speaker Negative Wire (-): Green

Hope helps (remember rated this) TY for use Fixya.

go to installer.com