Sure you can. I have a 12" in my '92 convertible (same trunk space) and could have fit more in there if I wanted to:
That's with the lesser space because of where my top folds down. You could mount your amps on the backs of the seats and use the trunk space for probably three 12" subs if you make a box to fit it right Get creative
the box i had was pretty small. subs faced towards the rear of the car. the box was somewhere around 36" long x 14" high x 15" deep if memory serves right. the crossbrace that was in that 88 coupe was bolted in. looked factory unless someone drilled the spotwelds out and just ran bolts thru it . been about 4 1/2 yrs since i had the car lol. but i do remember it being bolted in. was the 1st coupe ive owned so i dont know if others are the same way or not.
in my 93 mustang, i had a competition bandpass enclosure with 2 12's in it. now mind you, those enclosures kinda suck, but they fit great and actually couldn't be seen when the hatch was down.
You could fit several if you wanted to. The "selp amplified" ones won't put out nearly the sound or quality that you will get from the non-amplified ones (need an external amp), but a normal sub will be the best (in a proper enclosure, with the proper amp).
The box I have for a 10" woofer should fit in the trunk of a notch id think...its nothing all that special, stands about 14" tall i beleive. Heres how it looks...not a great pic but you get the idea...
btw dont mind the junky amp and ugly wiring...it all looks much better in my GT now lol