not sure you needed to start a new thread - where do i post?!
what model headunit do you have? my GT was fitted with a '937'.
check the label on the top of the headunit, which should look like the pic gotcha posted above. see what model headunit, and see if the schematic shows the 'booster in' wire. if it's a different model, and the wire is not shown, then it won't be designed to have it.
if it is shown, then consider tapping into another wire for it. but check first.
but, it should be obvious in terms of loudness/volume, whether it is required. because without the 'boost in' (on headunits that are compatible), the audio output signal is a higher voltage, like i explained earlier. and you would have very little 'sweep' of the volume control before it gets extremely loud, and have little resolution to control it. that is, from 1 to 2 would be a marked increase in volume, and you probably wouldn't get over 4 or 5, each increase making a big difference.
of course, normally you would use the volume up around 20~25, and each step makes smaller, incremental changes in volume.
A: if the volume control is normal, then the headunit is set and working correctly, and not designed for a booster-in wire; trying to add this in when it's not designed for it could damage it
B: if the volume is overly loud and crude, and it's a 937 model with a schematic showing the booster-in wire, then it probably needs it
you've made no mention of the volume control so far, so i'm suspecting you have a different model headunit to the 937 (ie: you have option A above), that does not require the booster-in wire, and is meant to have EQ settings. and i think this is a newer model headunit. if the car is an older model, then the headunit was probably replaced (eg: faulty original headunit, which isn't unheard of).