200SX Rear Speaker Access

First, make sure that your replacement speakers are 6.5"S (oversized) speakers. Standard 6.5" speakers will not fit the factory openings well. [Editoral note: The front speakers are basically the same as the rears. You can get a 6.5"S but not a 6.5". Current speakers are a 4x6 in a special bracket.]
Unbolt seatbelt from upper b-pillar.
Unclip rear window defroster plugs from window.
Pull off door-seal around paneling.
CAREFULLY pull out the lower rear passenger compartment paneling. As this overlaps on the upper paneling which must be removed.
CAREFULLY pull of plastic panel which covers the rear passenger window. The clips under this panel like to break, especially if cold.
Now that the back seat of your car is full of panels, move them around so that you can access the rear parcel shelf nicely, with out killing/scratching/damaging panels.
Loosen the clips holding the parcel shelf down, you might want to jam something in the holes (bits of rubber, rag, whatever is handy) so that the parcel shelf doesn't re-seat its clips back while trying to pull out the shelf.
PULL the shelf out. Your shelf might be glued in along the back edge from the window adhesive. Mine was really glued in and I had to really pull very hard on the shelf. You can also just bend up the shelf and remove and replace speakers with a stubby screwdriver, but this might crease it...
Remove and replace speakers.
Reverse procedure to get your car back to form.