 It may be more work than you think
Here's the deal, if you've broken the speaker cover off, then you probably realize the whole inside panel has to be removed. Furthermore, you'll need to take the rear seat out before you can take the panels out.
All in all, it's not a bad job if you've got some semblance of mechanical ability. It's just screws and things for the most part.
Before I go through the steps, let me make two suggestions before you start.
1 -If you don't have the Mach 460 sound system, find a junk yard or online Mustang site that specializes in parting out Mustangs, and buy yourself a set of the plastic boxes that the mach 460 rear speaker came in. Seriously... if you spent the money for decent speakers, this is a must!! The non-Mach 460 system has the speakers mounted to a free standing metal bracket, with no enclosure around the speaker. Repeat after me, "no enclosure, no bass". These boxes will be the single best investment in your convertibles sound system.
2- If you've been contemplating a rear seat delete kit. Now is the time to install it. You're going to have everything apart that would be required to remove the back seat and install one of these kits. I'm not advocating the kit, there are other forums for that, just mentioning that you'd save yourself a lot of time and double work if you did it now.
Ok, moving on, here's what you'll need...
Screw drivers (both phillips and regular)
Ratchet set with a set of torx sockets
Step 1 - Remove the plastic door sills (the pieces that run along the rocker panel when you open either door). They hold the edge of the carpet down. You'll need to remove them because the front pieces of the plastic panels are underneath the back few inches of them.
Step 2 - Feel under the bottom edge of your bottom back seat cushion. You should find two white tabs, one on either side. These are actually buttons that release the seat bottom. Push the middle of the plastic buttons and lift on the seat cushion to release the seat from the latches. I have found it more helpful to have someone else work one side while I do the other. It can be done with one person easy enough, but the first side tends to relatch quite often while you're pulling the second side out. Very frustrating. At any rate, once you release the seat, go ahead an lift the bottom cushion out and set some where outside the car.
Step 3 - Use the ratchet and torx socket to unbolt the two bolts holding the back cushion in place. You should see the metal tabs leading down from the seat cushion to the two large torx bolts that were hiddin under the seat bottom. Take those bolts out, lift upward on the seat back, and off it will come. Set that aside for later.
Step 4 - Now that all that is out of the way, you can get back to the actual panels. There are several sets of screws and maybe a bolt or two holding the panels in. Just work your way around each panel and remove anything holding it in place. Once it's loose, you'll have to pop it free. I seem to recall a few clips that hold it in place and a little bit of rubber near the quarter window. All of this should be fairly simple and obvious once you're in there. The only surprise here will be the seat belts. They are through the panels and will fight you when you try to move the panels away to reveal the speakers. You can leave them attached and just work around it, or I'd suggest removing the large torx bolt holding the top part of the seat belt which will free up the panel more for you.
Step 5 - Depends on whether or not you have the Mach 460 stereo system. If you do, your speakers will be in the black plastic boxes I mentioned earlier, if not, well there will be a free standing metal bracket with your speakers mounted to it sitting there. Go ahead and do what you have to do to get your old speakers out and get the new ones in, and then just reverse everything to put it back together. | 
