If you're lucky enough to found a 89-94 Tbird or Cougar with the relatively rare factory JBL system, chances are your luck immediately ran out the minute you turned the radio on for the first time.
This is usually why:
Now I won't go into details about what has clearly failed here, but to sum it up the round foam ring thingy attached to the cone shaped thingy and metal scaffolding has dried and cracked over the years. This is especially noticeable, even if the sub has been unplugged from the mix, because the factory JBL amplifier only sends the low signals to the sub and not the speakers giving a very thin bassless sound when listening to the radio.
I could get more technical here but suffice to say only the most boring tedious individual could understand the inner workings and proper terminology of a speaker. Let alone source replacement parts for and actually rebuild one!
Luckly though, even the most car audio inept person, such as myself, understands that the stock sub isn't worth repairing in the event of it's inevitable failure, so with some scouring a suitable replacement can be found elsewhere and can be installed in an afternoon.
-2-4 flat head screw drivers
-A couple sockets and a wrench - I believe 7mm, 8mm, 10, and 13mm
-Weather stripping adhesive
-Suitable replacement sub (6-8" will work with varying degrees of difficulty)
I opted for the JL 6W0 4 ohm. It cost around $100 and occasionally pops up on ebay and elsewhere as NOS. In my opinion, given the ease of install and the excellent performance compared to the 7" stock sub it's well worth it to grab one up for this.
First thing you'll want to do is discard the rubber mounting ring that comes with it and use the plastic one from the stock speaker. You can use weather stripping adhesive on the ring but I found it unnecessary for mine.
Next you'll want to separate the box halves. The factory seal isn't super strong but to remove it straight up like necessary you'll want to use flat head screw drivers on each side to gently pry up on evenly.
When separating the box halves try to remove it as straight up as possible. Reason being is the bolt side of the box (bottom half) forms collars around the square mounting posts on the other side of the box(top half). Any diagonal lifting can easily crack the posts upon disassembly. (Solution below)
Now that the halves are separated unbolt the stock sub and simply bolt in the new one. The only modification you will have to do is cut the speaker support shorter(it's 1/2" for the JL).
Make sure to reuse the stock foam isolator under the sub as well
Now that that's over with, If you cracked posts cracking during initial disassembly, which is easy to do, usually the cracks will appear in the last 1/2" to 1" of the post since the material is at the weak end of the taper. The good news is the posts are much longer than that and much thicker towards the bottom(thus less susceptible to cracking) so the problem is easily solved with with these few extra steps:
-4 longer self tapping screws (depending base the length on how much you cut the posts down to) I'm not sure what the thread size/pitch they were. I just took mine to the local ACE and matched the stock ones up.
-4 nylon spacers (again base size on post cut length) I used 1"
Here you can see the stock posts vs. the cut down posts
I cut by the length of the nylon spacers and wrapped masking tape around the post to give me the line to cut from. I used an open hacksaw to cut them and smoothed it with 100 grit sandpaper.
I also drilled the holes deeper for the longer screws.
Here I added the nylon spacers to the bolt side.
The spacers were actually a perfect fit for the collars in the enclosure. They held in snug and make for super easy installation.
Before that though you'll want to put the insulation mat back in place and more importantly, wire in the speaker. The stock terminals plugged right in.
Now seal the box halves with weather stripping adhesive and reattach them. Be very careful not to overtighten the screws, especially if you had to use the longer ones. You'll want to tighten and loosen them little by little so as to properly tap the holes.
From here reinstall the box into the car and enjoy!