You have a few choices now.
First, build some custom enclosures in the back (above the cubby holes behind the rear arches is where i'd suggest ) and fit a pair of 6x9s or similar larger speakers. Run dedicated speaker wire to these.
Second is find a pair of extra slim 5.25"/13cm door speakers and fit them in your rear doors, again running dedicated speaker wires for them.
Third is a normal pair of 5.25"/13cm speakers, drill out the spot welds on the blanking panels in the rear doors and remove the blanking panels. If the wiring is hidden back there (usually it's folded back and taped to the rest of the loom so might need some hunting) connect it and see what you've got. You may still need to run separate wires though.
I've not had a 7xx without the rear door speakers so no idea if they fitted the wiring on models that didn't have rear speakers. However, how often do you have rear seat passengers? If the answer isn't often, i'd go for the custom enclosures at the back of the car. Simple reason being, the rear speakers in the door don't contribute greatly to what you actually hear - there are two big seats directly in front of them to instantly muffle their output!
I have a pair of 6x9s that i keep meaning to fit and wire up at the back, perhaps through an amp (nothing lairy like a mega-watt amp, just enough to give a bit more oomph from the back) to give a better all-round music image.
Good luck, whichever way you do it!
