To the OP:
I started installing car audio in the late 1970s and was around it until 2000. Recently got back into the hobby as I miss it and have been restoring my Foxbody vert since 2010 as well. Owned a GT hatch prior to that since 87; I competed, was an IASCA judge and also worked for Pioneer Electronics as Product Manager for the car audio division for 6 yrs. Have built hundreds of decent sounding mid level systems, which is the most satisfying - giving customers more than they expected.
Now, Fox vert......that's a real challenge, since the car is noisy inside, the windows likely leak air, the exhaust is loud and there are limited good OEM locations for speakers........especially subs. Good equipment installed incorrectly will not sound good, and crappy equipment installed properly has it's limitations.......
First things first. Best dollar value and result will be to use a component setup in the front, with a woofer in each door and a tweeter in the dash; all OEM locations. Fitting the largest diameter woofer pays real dividends, as does Dynamatting and Dynalinering each door fully and properly. An outboard passive crossover for each channel is also needed.
A real amplifier is not an option, it' an absolute necessity, as is a decent signal source, ideally a CD, but high quality digital files can be used as well.
That's what I would start off with; either a 5-1/4" or 6-1/2" (if you can trim the door metal cutout slightly) component system. Amp wise, you're going to want 50 watts rms per channel across the bandwidth at below 1% distortion.
I would try that for starters, including trying wiring both tweeters out of phase with the woofers to see if it clears up the mids, since they are on a different plane than the door speakers. Set the amp gains up so there is no distortion at all at full signal level, this will ensure you don't blow up the woofers (distortion kills speakers, not power at the levels we're talking about).
Obviously, the amp needs direct power from the battery, a good ground and half way decent RCA cables from the source unit.
Once this is done and you want more, the next thing is going to be some kind of sub. Easiest is a Bazooka tube strapped in the trunk or on the floor behind one of the front seats if you are ok with that. Ghetto, but it works and it's cheap. Otherwise, you can build a front wall and use the entire trunk as a bass enclosure ( I did 4 tens that way in my ex gfs Fox coupe a few years ago), you can build a box and have it sit in the trunk, or you can raise the floor and use the spare tire well for a ten or a 12 or you can build a rear seat delete like I did with a pair of 15"s lol. You will also need another amp and electronic crossover (built in or separate) for this portion of the emerging build. If you have an old school 2 channel amp (like a Punch 45 or Orion HCCA or SX) running the fronts,you can passively make it a 3 channel and run the sub bridged and the front still stereo. Need caps and coils and some tuning skills tho.
I believe the first stage is what you should do, ensure it is tuned and setup correctly. I've bought a bunch of vintage car audio equipment the past 5 yrs,; you should be able to find a speaker setup for $100-200 new or used, depending and an American made amp like an Orion or Rockford for maybe $100-200. Add another hundred or two for proper Dynamat and Dynaliner, fuse holder, power and RCA cables etc and you're off to the races.
After the sub, it would be rear fill in the 5x7 or 6.5" OEM rear locations, depending which trim level and year the convertible is. That isn't nearly as critical tho. I've attached some pics of my current vert stereo build. It's using Boston Acoustics 6.5 Pro front components, Dynaudio / Amega rear fill, Dr. Crankenstein 15 and 10 inch subs and midbass, all run with either Orion HCCA or Rockford power (havent gotten that far yet).
Hope this hard earned wisdom helps you and any of the interested readers out a little bit.