You can tap into and run wires from the back of the head unit. The wiring diagram on BOSE head units from 2007 on is as follows
solid blue (remote turn on)
solid purple (FL+)
green w/ white paint (FL-)
solid red (FR+)
solid green (FR-)
brown w/ blue stripe (BL+)
yellow w/ blue stripe (BL-)
yellow w/ green stripe (BR+)
white w/ blue stripe (BR-)
This was certified CORRECT on my 2008.5 MS3. If you do not have these color wires, or they are separated in some weird way, you may have to find a different diagram.
You will need a power wire as well, I ran it behind the glovebox through the rubber gromit. The glovebox is held in by two black screws that you will find behind the carbon trim above it. The carbon trim is clipped in but be careful as it hooks in on the passenger side. After the screws that hold it in are out, the glovebox will snap out with a few gracious heaves (I pulled on the latches of the glovebox door).
Run power down one side of the car, speakers down the other. You will need an amp with either high level inputs or use a line out convers/line driver to convert to RCA. All the panels pop out - the rocker covers are clipped in, and there is on pressure clip holding the kick panels in (I used an exacto knife to pop it off). Getting behind the rear seat will take some creativity.
You may not be entirely satisfied with the volume you will get even with your amp cranked on gain and bass boost, you might have to use a line driver to boost the signal. The volume level will only ever get as loud as the front or rear speakers at maximum, and the sound has a longer way to travel. Also, the factory deck cheats you out of some audible frequencies - anything below 80hz seems to be deeply attenuated. There isn't much you can do about this except fiddle with your crossover settings or buy some kind of sweeper or EQ flattening device, or replace the factory deck with an aftermarket one.
