Wiring harness problem

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

https://www.modifiedlife.com/2004-pontiac-grand-prix-stereo-wiring-diagram/

You need to go to your local "best buy" or electronics store and ask them for your car's "stereo wire harness". This will convert the wires from your car to match all the colors from your deck. (green, purple, white, gray, blue, yellow, red, ect. Same on all after market decks. All brands use the same wires) Take that new harness and your deck harness, twist the matching wire colors together, tape them with electrical tape, plug the harness back into the car and deck, and it should be good. Click, click, done. Remember to plug the antenna in as well before you get the deck in.

you just go by your harness wires and colors

1994 Jeep Grand Cherokee Car Radio Stereo Audio Wiring Diagram
Car Radio Battery Constant 12v+ Wire: Pink
Car Radio Accessory Switched 12v+ Wire: Violet/White
Car Radio Ground Wire: Black
Car Radio Illumination Wire: Orange/Black
Car Stereo Dimmer Wire: Green/Red
Car Stereo Antenna Trigger: N/A
Car Stereo Amp Trigger Wire: N/A
Car Stereo Amplifier Location: N/A
Car Audio Front Speakers Size: N/A
Car Audio Front Speakers Location: N/A
Left Front Speaker Positive Wire (+): Green
Left Front Speaker Negative Wire (-): Brown/Red
Right Front Speaker Positive Wire (+): Violet
Right Front Speaker Negative Wire (-): Blue/Red
Car Audio Rear Speakers Size: N/A
Car Audio Rear Speakers Location: N/A
Left Rear Speaker Positive Wire (+): Brown/Yellow
Left Rear Speaker Negative Wire (-): Brown/Blue
Right Rear Speaker Positive Wire (+): Blue/White
Right Rear Speaker Negative Wire (-): Blue/Orange
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The easiest thing to do would be to purchase a Metra wiring harness. It plugs right in to the plugs that you will remove from the factory stereo. This simplifies wiring, as you can crimp wires by matching colors on the harness instead of mixing and matching colors on the factor wires. You can purchase this harness at PepBoys or other places where they sell car stereos. The harness looks like this. I used this harness on my 94 SL2 the same model harness applies to yours

Get a wiring harness adapter kit (about $10) from a stereo shop. It has harness plugs that will plug directly into your harnesses in the dash, and has color-coded wires that you will color-match to the wires on the aftermarket deck's harness. You may need an antenna adapter as well. If your stock stereo face is taller than the aftermarket one, you'll need a dashboard adapter kit to take up the extra space.

best advice is to go to a stereo store or a auto parts and by a harness kit. this way you sit at a desk and match up wires,crimp em together, then just plug them in. compare the schematics of deck and kit and thats it, there like 15-20 bucks and you dont cut any factory wires to install.