Well, I might be a little late for this, but I'll put in my two cents as a fellow '93 SC2 owner who has swapped rear speakers in the bitter cold.
Do what picksl1 said. Warm the sucker up, go for a drive, whatever, just get some heat going. (I suggest warming up the car with the fold-down rear seats in the down position so the trunk gets some heat in to it.)
Next, get yourself a flashlight or droplight and crawl on your back head-first in to the trunk from the back seat. it's a bit awkward, but it seems to be the best position.
I don't believe that you need to remove the speaker grilles to remove and replace the speakers, but it has been a year since I did this and I have a hard time remembering yesterday.
If you need to remove the grilles, there will be four round speed clips on each grille, visible from below. The honest to God easiest way to remove them is to just grab them with a pair of needle-nose pliers and give them a twist to break them. You can try salvaging them, but it will take forever and chances are they will break anyway.
If you don't have to remove the grilles, and I don't think you do, there will be fasteners accessible from below the package shelf.
The wiring to each speak unclips pretty readily.
Everything else is pretty self-explanatory.