so...your car is the sedan? and speaker size you bought is...?
if you're wishing to install them on the parcel shelf, you will need to customise the entire installation:
*cut suitable size hole in parcel shelf trim
*build suitable mdf wood (or other material) spacer rings/baffles
*run new speakerwiring
there's a cut-out on the shelf, as if to hold speakers. but this area is not perfectly flat. and the trim is not really solid enough to hold speakers properly.
when i added speakers here, i cut the hole, and mounted the speakers underneath on custom mdf wood baffles/'shelf'. then built my own grills.
i've got pictures amongst these: : Gallery
easier, is to fit any new rears in the rear doors where the factory ones are, and power them off the headunit. otherwise, do you have an amplifier to run them?