If your wanting to wire into the premium sound plug(gray data cable), I would recommend not to. Instead use the factory amp bypass plugs. Interfacing the factory amp with an aftermarket headunit can create unwanted turn on pops, humming, and whining through your speakers. The factory amp bypass plugs are located directly behind the A/C controls. They are held by a plastic clip, so just pull them down. unplug both harnesses and your left with the standard Ford factory radio plugs. You can pick up a set of smart harnesses from Circuit City, so you don't have to do any wiring. The vehicle side of the
plug is a Metra 801770, and other side will be your new stereo plug with a universal connector that will link the two harnesses together.
There no way to get it wrong.
If your bypass plugs are the ones that are already cut then you'll have to hard wire in your radio harness. Whitepony92 has the correct colors
listed for you. The battery trick works great for matching speaker wires, but to find out which wire is positive or negative you will probably want to use a 9volt and watch which way the speaker moves when it is pulsed. If the speaker moves outward, you have the correct polarity. If the speaker moves inward, you are out of phase, just reverse the wires. Phasing is important because if you don't have it right, your bass frequencies will cancel out causing the bass to sound hollow or like your head is in a bucket. I believe your positive speaker wires are going to be white w/ green stripe, orange w/ stripe, pink w/ green stripe, and pink w/blue stripe. I would check with the battery just to be sure.
Hope something in there helps. interceptor out.
__________________ 1990 police interceptor coupe. | 
radio wiring diagram 
I went to put a new stereo in the stang and found out the guy before me just cut all the wires. I usally don't have a problem with this but the radio will not keep memory. I was just wondering if anyone has a wiring diagram for the Premium Sound System for a 90 GT. Thanks
