The first pic apears to be a 'Mini ISO' used as a line out for feeding an amp, like on cars with Bose.
They are normally in three bits that pull apart, yellow, green and blue... one bit for changer, one for phone, and one for line out, but looks like its all in one, but only one end is used.
Second pic shows the two 'ISO' plugs, the black one is the power, and the brown one is for the speakers... but on the speaker one only 4 of the 8 pins are used, so you are correct only the fronts are fed direct from the stereo.
I would guess there is a factory fitted amp in the back of the car, in my Saloon with Bose it is on top of the drivers side rear wheel arch.
Third pic looks like a 'Fakra' which is normally used for aerials, or GPS, etc... is there a more normal aerial type plug there aswell? If so it could just be left from some phone prep kit that has been in there in the past.
I dont really know what your options are, such as if you can feed the amp using the line out on the back of a normal radio, never tried to do it, but someone must know.