While not a GT, in my hatch, I just have a couple of those wedge-shaped 6"x9" boxes with some old Pioneers in there. I had the same speakers mounted in the stock location in the side panels in the hatch area (I know the '87+ hatches had 'em farther up on either side of the rear seats), but they just buzzed a lot and had ZERO bass and barely any midrange to 'em. I don't have an amp on them at all, but they sound plenty fine with good enclosures.
If I had some extra cash to blow, though, I'd make use of the wedge-shaped enclosure I have for a pair of 10" subs and then just screw the amp down onto the back side of the enclosure and put it up near the rear seats. However, I don't have a hatch curtain like Lynn does in her Capri to hide the system (can't find one for a four-eyed Fox anywhere), and I don't want to limo-tint the rear because that usually just screams, "Hey, everyone! I'm trying to hide some stereo stuff in here! Break into my car, please!"
Also, because my '84 is my DD I can't justify taking up the entire cargo area with stereo junk - the 6x9 boxes at least allow me to shuffle things around to make more room if needed, and I still have plenty of grocery space even with the rear seats folded up. If I got a major boner for more sound, I might just lay a small amp between those two 6x9's and then plug my other pair of 6x9's in the stock panel holes, but ... nah.
FWIW, Lynn's Capri has a generic no-name brand sub box with a (I think) 200-watt Jensen amp hooked to it, and it actually sounds pretty good, IMO - not overpowering, but enough that you can feel it and annoy old people as you drive by.
And it's good enough for the kind of music her and I listen to, anyway (metal, mostly), but if you're into that BOOOOOM BAH-BOOOOOOM bass-heavy hip-hop stuff, then ... probably not what you're looking for.
. I have a 5 switch panel in place of the radio. Bought an antenna delete for the fender. Who needs a radio?
