[evansc22]

I have a 1991 GT. It has the factory radio with EQ installed.

I have purchased a wiring harness kit (FM-K504), but only the power side plug matches up with what's under the dash. The speaker wire plug will not fit. I'm at a loss so far.

I referenced this wiring diagram information, and couldn't locate all the colors mentioned:

Radio (Modified Cars, Car Radio Wiring Diagrams, Car Alarm Wiring Diagrams, Car Light Bulb Size Guides - ModifiedLife.com)
1991 Ford Mustang Car Stereo Wiring Guide
Car Radio Battery Constant 12v+ Wire: Green/Yellow
Car Radio Accessory Switched 12v+ Wire: Yellow/Black
Car Radio Ground Wire: Chassis
Car Radio Illumination Wire: Blue/Red
Car Stereo Dimmer Wire: N/A
Car Stereo Antenna Trigger Wire: Orange/Blue
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 (+): Orange/Green
Left Front Speaker Negative Wire (-): Light Blue/White
Right Front Speaker Positive Wire (+): White/Green
Right Front Speaker Negative Wire (-): Green/Orange
Car Audio Rear Speakers Size: 5″ x 7″
Car Audio Rear Speakers Location: Side Panels
Left Rear Speaker Positive Wire (+): Pink/Green
Left Rear Speaker Negative Wire (-): Pink/Blue
Right Rear Speaker Positive Wire (+): Pink/Blue
Right Rear Speaker Negative Wire (-): Green/Orange

I know for sure he EQ did not come stock in this car. I'm wondering if there exists a harness that will plug directly into the smaller plug.

Please see pictures located here --> Mustang Radio Install - a set on Flickr

[evansc22]

After reading another post that was somewhat similar in nature, and contacting one of my local friends, I was able to locate the plug for the speakers.

For others that may run across this and need the help, the plug for the speakers is behind the climate control knobs to the right. You have to bypass the stock amp (I guess that's what the silver box is directly behind the radio, and mounted to the floor). The plug is very hard to get to. I had to put one hand through the climate control area (had to remove that), and one hand/arm wrapped under the dash from near the glove box, and had my face planted into the dash...and was able to get it unplugged.

Once I crossed that hurdle, the rest was fairly self explanatory. I really hope this helps some other poor soul......

[EagleAutosports]

Replacing the stock radio isn't just remove the old and stick in the new. There's lots of wiring that has to be changed, sometimes even if you have the harness connector.