I have an 1989 GT Convertible with Premium Sound (6 speakers and the amp behind the Headunit).
I still have the Single CD Tuner from my previous 1995 GT with the MACH 460 setup (looks like your pic in post #7).
Do you know if this fits in my 89 GT?
No. You'd have to change to the 1993 style amp to be able to hook it up.
Another thing to think about is how to mount it