One of the least important mechanically, yet most important comfort-wise, is the sound system.  I have 3 major problems with mine.
1. The stock radio has "problems" ranging from intermittent functionality to varying volume (independent of the controls) to the lack of a CD player.  I will be replacing this as soon as $$ allows.
2. The speakers are not right.  The passenger rear sounds blown and the 2 front speakers sound odd.  Not sure why.
3. The Subwoofer is blown.

My first order of business was to replace the standard speakers all around.   Considering my exhaust setup (No muffler w/resonator tips) I accepted the fact that I would never achieve concert quality sound out of my sound system and went with the factory replacement speakers from AutoZone.  4 Speakers for around $50.  They won't win any awards, but they sound fine.  The Radio will come next, then the subwoofer.  Below is the process I used for the speakers.

REPLACING THE SPEAKERS IN AN 87 GT.
Parts list
13mm Socket (I think)
T-47
Philips Screw Driver
4X10 Speakers (front)
4X6 Speakers (Rear)

1. To replace the rear speakers you need to remove the molding by the seat.  For this you will need a Philips Screw driver and a T-47 Torx bit.
For clearance I also removed the seat.  To take out the seat remove the 4 nuts under seat.  Move the seat all the way to the rear to reach the front ones with a ratchet.

and all the way forward for the rear ones.

When you remove the seat you will find this is an amazing place for trash to accumulate.   I found spark plugs, leaves, wrappers and a floppy drive!

2. Then remove the Philips Screw in the door panel. And unlatch the cover over the seat belt bracket.  Press it out from behind (there is a joint at the top that holds it together.)
 
3. Then remove the Torx Bolt (T-47) that holds the top pivot point in place.

4. With this out of the way the entire panel can be slid off to the side.  It shouldn't be necessary to remove the belt from it.

5. The 4X6 speaker can then be accessed.  Remove the screws and gently release the clips.  Mine were already broken and judging by the cobbled wiring and issues below, it appears as though someone attempted to replace the speakers in this car before.

Install the new speaker in its place.

I only had one problem with the rear speakers.  Because my clips were broken I had a low frequency vibration when passing mid to high volume through the speaker.  In order to correct this I had to add more hold-down points on the speaker.  I used my handy-dandy drill to provide these as the frame wasn't previously setup for this.   It's much better now!.

6. Then It was time for the front speakers.  To remove the cover, GENTLY/CAREFULLY pry the speaker cover up.  The dash is prone to cracking when this is done. (mine did)

7.  When I pried mine up I found out why they didn't sound right.  Here is what was installed. (2 pictures Passenger, 1 Driver)

8. <shudder> there were 5" rounds in place of the stock 4X6.  They were held in place with a piece of copper wire through the original mounting holes.  With these removed I installed stock 4X10's.  Fortunately the speakers I got had new mounting clips included to replace the missing ones in the dash.

9. Pop the cover back in place and you're done!
10. I also used the opportunity of the seat being out to install a cheap-o seat cover to prevent my seat from being damaged further than it was. It's a lot easier when its out.