Well this is what I have done and I thought I should post how to do it to help out my fellow Mustang lovers. Its very simple infact takes maybe 2 hours? Took me 5 hours to figure out the wire configuration but I did and will pass that information on to you.
-You need to buy a Stock Ford Cd Player (Sony) from a 94 - 99 GT and possibly look for one with a wiring harness (Usually they give them away because they remove it all.). If you cannot find on then I suggest
www.stangparts.com ...They have some for $400.00 and say they will go with your car, but in the real world they wont work without tearing your car all the way to the frame

. You can use the 1999 GT's Audio Harness...There something like $50.00?
-By now if you have wanted to add a cd player you probably already have had the Cassette and that little black tray out. Well notice on the back of the Cassette there this huge plug going into the pack of it. About in Inch long. This plug is going to be the brains for this whole cd player so this is what were going to work with.
-You need some Quick Snaps from K-mart or Walmart doesnt matter. They will make the wire splicing so much easier and help out. Before we start anything I ask that you take the negative battery cable out so you do not get sparks flying by touching the two grounds together.
-Take your 1999 plug and undo all the tape to expose the wires attached to the RED plug that will fit into the back of the cd unit.
-(MAIN PLUG GOING INTO CASSETTE) Now looking at the plug please locate the following on the back of the cassette plugs. These are wire colors.
Blue, Green:Black, Light Blue, Yellow:Black, Black, Green
-On the 1999 GT Cable locate the following colors:
Green:Black, Light Blue:Red, Orange:Blue, Black, Green:Purple
Ok now its time for the quick connects to do there job. Use the quick connects and connect the following two wire colors together. Some wires may not be used. Blue isnt used.
CONNECT THE FOLLOWING:
Blue - (Amp Trigger) Not needed.
-Cassette Wire- -1999 GT Harness Wires-
Green:Black - Green:Black (ground)
Light Blue - Light Blue:Red (Illumination Dimmer)
Yellow:Black - Orange:Blue (12 V Ign)
Black - Black (Ground)
Green - Green:Pruple (12 V Constant)
I hope this works out for you! It took me about a day to get working. I had to sit there and test everything to make sure everything went together. IF you have any questions concerning this or need help please let me know. You can either PM me or E-mail me at
Fast1995GT@yahoo.com 

Thanks~!
-Mack Daddy